Safe take-off with runway analyses

Content

List of figures

List of tables

List of abbreviations

introduction

1 Definition and purpose of runway analysis

2 Factors affecting MTOM and V-speeds
2.1 MSTOM
2.2 The aerodrome runway distances
2.2.1 Line-up distance
2.2.2 Take-off Distance
2.2.3 Take-off Run
2.2.4 Accelerate-stop Distance
2.3 Climb limitations
2.4 Obstacle clearance
2.4.1 Vertical plane
2.4.2 Horizontal plane
2.5 Meteorological elements
2.5.1 Wind
2.5.2 Pressure altitude
2.5.3 Temperature
2.6 Runway slope
2.7 Runway condition and contamination
2.7.1 Definitions
2.7.2 Effect on aircraft performance
2.8 Tire speed limit
2.9 Brake energy capacity
2.10 Aircraft configuration and systems setting
2.11 Aircraft status
2.12 Bearing strength

3 Take-off data optimization principle
3.1 Aircraft configuration and systems setting
3.2 v1/vr ratio
3.2.1 v1/vr range
3.2.2 v1/vr ratio influence
3.3 v2/vSR ratio
3.3.1 v2/vSR range
3.3.2 v2/vSR ratio influence
3.4 Take-off data determination
3.4.1 MTOM
3.4.2 v-speeds

4 existing runway analyses products
4.1 Aircraft manufacturer software
4.2 APG
4.3 Flygprestanda
4.4 ASAP
4.5 Honeywell
4.6 EFRAS
4.7 Conclusion of runway analyses review

5 runway analysis conceptual model for Take-off
5.1 Input data
5.2 Mass to v1/vr ratio graph
5.3 Optimization process
5.4 v-speeds evaluation

Conclusion

Bibliography

Appendix

LZKZ – Košice

Ambient weather conditions

ATR 72-

Mass to v1/vr ratio graph

Optimization process

v-speeds evaluation

Conclusion

List of figures

Figure 1: Declared distances

Figure 2: Line-up distance correction

Figure 3: TOD one engine inoperative

Figure 4: TOD all engines operating

Figure 5: TOD one engine inoperative wet conditions

Figure 6: TOR one engine inoperative

Figure 7: TOR all engines operating

Figure 8: TOR one engine inoperative wet conditions

Figure 9: ASD one engine inoperative

Figure 10: ASD all engines operating

Figure 11: Take-off path segments

Figure 12: Take-off flight path with obstacles

Figure 13: Departure sector (Track change ≤ 15°)

Figure 14: Departure sector (Track change > 15°)

Figure 15: Runway slope

Figure 16: Aquaplaning phenomenon

Figure 17: Take-off flight path on a wet and contaminated runway

Figure 18: Optimum flap setting

Figure 19: v1/vr effect on runway limited MTOM

Figure 20: v1/vr effect on climb, obstacle, brake energy and tire speed limited MTOM

Figure 21: v2/vSR effect on runway, brake energy and tire speed limited MTOM

Figure 22: v2/vSR effect on climb, obstacle limited MTOM

Figure 23: Optimum MTOM and v1/vr ratio

Figure 24: MTOM as function of v1/vr and v2/vSR

Figure 25: Take-off speed calculation

Figure 26: Airbus LPC take-off interface

Figure 27: TAP Portugal TLP take-off optimization mode interface

Figure 28: TLC take-off chart example

Figure 29: OCTOPUS take-off chart example

Figure 30: APG take-off chart example

Figure 31: APG performance data tablet format example

Figure 32: Flygprestanda take-off chart example

Figure 33: ASAP take-off chart example

Figure 34: Honeywell take-off chart example

Figure 35: EFRAS input section example

Figure 36: EFRAS Main output menu example

Figure 37: EFRAS Details output menu example

Figure 38: Mass to v1/vr ratio graph flowchart

Figure 39: Optimization process flowchart

Figure 40: v-speeds evaluation flowchart

Figure A1: LZKZ aerodrome chart [Source: AIP Slovak republic]

Figure A2: Mass to v1/vr ratio graph flowchart

Figure A3: 2nd segment climb gradient limited mass

Figure A4: Final take-off segment climb gradient limited mass

Figure A5: TOR corrections

Figure A6: TOR 1 engine out limited mass

Figure A7: TOR all engines limited mass

Figure A8: TOD corrections

Figure A9: TOD 1 engine out limited mass

Figure A10: TOD all engines limited mass

Figure A11: ASD corrections

Figure A12: ASD limited mass

Figure A13: Rwy 01 MTOM vs. v1/vr ratio graph

Figure A14: Rwy 19 MTOM vs. v1/vr ratio graph

Figure A15: Optimization process flowchart

Figure A16: TOD rwy01/19

Figure A17: TOD rwy01/19 corrections

Figure A18: LZKZ obstacles

Figure A19: Obstacle clearance

Figure A20: Turn radius

Figure A21: Bank angle c. grad. decrement

Figure A22: 2nd segment climb gradient adjusting

Figure A23: Rwy 01 MTOM vs. v1/vr ratio obstacle corrected graph

Figure A24: TOD rwy01 adjusting

Figure A25: TOD corrections adjusting

Figure A26: Obstacle clearance adjusting

Figure A27: v-speeds evaluation flowchart

Figure A28: vr evaluation

Figure A29: v1 limited by max. brake energy

Figure A30: Rwy 19 MTOM vs. v1/vr ratio graph

Figure A31: TOD rwy 19 adjusting

Figure A32: TOD corrections adjusting

Figure A33: Obstacle clearance rwy19 adjusting

Figure A34: v1 limited by vMCG

Figure A35: vMCA

Figure A36: v2

List of tables

Table 1: Take-off segments characteristics

Table 2: Minimum climb gradient requirements

Table 3: Contaminants categorization

Table 4: Parameters influencing take-off

Table 5: Influence of v2/vSR ratio change on take-off limitations

Table 6: Input data for take-off calculation

List of abbreviations

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Introduction

Mainly world economic crisis has proved that the airline industry operates at precarious marginal profits and all companies in this sector need to pay attention to keep balance between their revenues and costs. Although the take-off and landing represent only a small portion of the total operation of an aircraft, performance of these two phases is considered to be very important due to entirely different reasons(1). Based on effort to become more competitive, the airlines struggle to operate at maximum possible payloads. On the other hand, both the take-off and landing are the most strictly regulated segments of a flight. Therefore, an aircraft performance optimization must take place to ensure safety level required by the regulation and also to allow the most effective way of airline operation.

An objective of this book is to study all relevant factors, which could influence the aircraft performance during the take-off phase of a flight, to describe an optimization principle and to propose a calculation method to obtain the maximum operationally allowable mass of an aircraft. This work focuses to do it with accordance with currently valid regulation of the European Union.

Nowadays, the regulator within the area of the European Union is the European Aviation Safety Agency (EASA) together with the European Commission. The EASA lays down the certification requirements for all aeronautical parts. Considering the large transport aeroplanes, formerly used regulation JAR-25 has been replaced with CS-25 with direct force of law for all EASA members. Binding procedures affecting the aircraft operation are specified in EU-OPS commended by the European Commission, but it is estimated that by the end of 2012 this regulation will be also transferred under the responsibility of the EASA(2).

As the optimization to obtain the maximum masses for take-off is forced mainly by economic reasons, this work covers only large commercial airplanes, which are assigned in a performance class A. Into this class, according to EU-OPS, belong multi-engine aeroplanes powered by turbo propeller engines with a maximum approved passenger seating configuration of more than 9 or a maximum take-off mass exceeding 5 700 kg, and all multi-engine turbojet powered aeroplanes.

1 Definition and purpose of runway analysis

One of the most essential responsibilities of an aircraft operator is to ensure a safe operation according to the authority regulations as well as manufacturer’s instructions. Considering the fact that the take-off and landing are the most demanding and dangerous phases of a flight, the operator is specially required to pay attention that all pre-flight calculations for these phases are fulfilled. Moreover, the output data of these calculations need to be perfectly understood by all relevant members of a flight crew, who directly conduct the flight, and a flight dispatching, who uses these data during the planning of aircraft activities.

Besides other factors, the Maximum Take-off Mass (MTOM) at a particular aerodrome for specific local and weather conditions is one of the most limiting figures for the aircraft take-off phase. This mass must be reached at latest on the runway at the time of brake release at the start of take-off. According to EU-OPS, an operator shall ensure that the take-off mass does not exceed the maximum take-off mass specified in the Airplane Flight Manual (AFM) for the pressure altitude and the ambient temperature at the aerodrome at which the take-off is to be made (3). Consequently, the lower figure of the Maximum Structural Take-off Mass (MSTOM) and a calculated maximum performance limited mass for the take-off for specific aerodrome conditions is the MTOM. For a safe aircraft operation it is always necessary to consider the worst case scenario and therefore a performance limited mass for the take-off must also include a possibility of critical engine failure at the decision speed v1. For that reason, in addition to the MTOM, the appropriate take-off speeds including the decision speed need to be determined.

To determine a performance limited mass most of the operators used to use so called aerodrome tables or tables at manuals provided by the aircraft manufacturer (QRH – Quick Reference Handbook). These data though tend to be very conservative and therefore there is a tendency of the aircraft operators to determine a performance limited take-off mass by using software for so called Runway Analyses(4). The aircraft performance data calculation reflects through the whole airline operation and that is why the airlines nowadays either subcontract or use their own tailored software to optimize the necessary take-off performance calculation.

Similar to the MTOM it is necessary to determine the Maximum Landing Mass (MLM) too. An operator shall ensure that the landing mass of the airplane does not exceed the maximum landing mass specified for the altitude and the ambient temperature expected for the estimated time of landing at the destination and alternate aerodrome(3). This mass is the lower figure of a Maximum Structural Landing Mass (MSLM) stated by the manufacturer and a maximum performance limited landing mass calculated for specific conditions at the destination aerodrome according to the AFM. For a determination of the performance limited landing mass the aerodrome tables, the QRH tables or specialized software for runway analysis are used as well(4).

As a conclusion of above mentioned statements it could be defined that the Runway Analysis facilitates the determination of the maximum allowable take-off and landing masses with associated speeds, based on critical engine failure, for specific airport/runway conditions and various airplane configurations. The limitations observed are those specified in the Civil Aviation Authority approved AFM for the airplane. (5)

2 Factors affecting MTOM and V-speeds

The following chapter describes all factors which could affect the determination of the MTOM and associated v-speeds. A strong relationship between them supposes that there cannot be made a sharp line between those factors affecting the MTOM and those affecting the v-speeds. These figures go hand in hand and determination of a proper value of the MTOM for specific conditions without calculation of the associated v-speeds, and vice-versa, is impossible for a safe operation of the aircraft.

For the purpose of this book v-speeds are defined as speeds associated with a take-off phase of a flight. Regarding operational aspect mainly following speeds belong to this group; however there exist many others as well:

- v1; take-off decision speed - maximum calibrated airspeed established by the operator at which the crew can at latest decide to reject the take-off and it is ensured to stop the aircraft within the limits of the runway
- vr; rotation speed – calibrated airspeed at which the pilot initiates the rotation during the take-off to reach v2 at 35 ft height above the runway surface
- v2; take-off safety speed – minimum climb speed in terms of calibrated airspeed that must be reached at a height of 35 ft above the runway surface in case of engine failure

2.1 MSTOM

A Maximum Structural Take-off Mass is a maximum permissible total aircraft mass at the start of the take-off run specified in the AFM or operations manual (OM), if more restrictive. This mass is determined in accordance with a flight structure resistance criteria and a resistance of landing gear. An aircraft operator must ensure that the take-off mass (TOM) at the brake release never exceeds the MSTOM as it could result in a damage of the aircraft structure. However it could be equal or lower than the Maximum Ramp Mass (MRM), to allow for a fuel used in start-up and taxi.

2.2 The aerodrome runway distances

Runway length, as one of the most important runway characteristics, influences aircraft operation on a particular runway considerably. Annex 14, Volume I, calls for a calculation of declared distances for a runway intended to use by international commercial air transport, and Annex 15 calls for a reporting of declared distances for each direction of the runway in the State Aeronautical Information Publication (AIP)(6). These declared distances for each runway of a particular aerodrome could be found in part Aerodromes of the AIP. Regarding the take-off these declared distances are:

a. Take-off Run Available (TORA); the length of runway declared available by appropriate Authority and suitable for the ground run of an aircraft taking off;
b. Take-off Distance Available (TODA); the length of the take-off run available plus the length of the clearway (CWY) available, if provided;
c. Accelerate-stop Distance Available (ASDA); the length of the take-off run available plus the length of the stopway (SWY), if provided.

The introduction of SWYs and CWYs and the use of displaced thresholds on runways created a need for accurate information regarding various physical distances available and suitable for the aircraft take-off. Following figure illustrates examples of different configurations of declared distances for various layouts of SWYs, CWYs and displaced thresholds at the aerodrome.

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Figure 1: Declared distances [Source: ICAO, Aerodrome design manual, Part 1 Runways]

According to EU-OPS 1.490, an aircraft operator is responsible for proving that distances required by the aircraft for a safe take-off, respectively aborted take-off, do not exceed the appropriate declared distances available at the departure aerodrome. Moreover, when showing compliance with above mentioned, an operator must take into account also the loss, if any, of runway length due to an alignment of the aircraft prior to the take-off.

2.2.1 Line-up distance

Prior to the take-off it is necessary to align the aircraft with runway heading in a direction of intended departure. To utilize all runway length it is favorable to do so at the very beginning of the runway surface dedicated for the take-off. At most aerodromes turns on the runway are required and therefore any time the access to the runway does not permit positioning of the aircraft at the threshold, the line-up corrections are necessary to consider. The alignment distance depends on the aircraft geometry and the access possibility to the runway in use. Usually, it is required 90° turn in order to enter the runway from a taxiway or 180° turnaround on the runway. Typically, the minimum line-up distance is provided by an aircraft manufacturer, but in case of 180° turnaround the minimal runway width, required for performing such turn, has to be additionally considered.

Consequently, the aircraft line-up distance at a particular aerodrome negatively influences the distances of TODA, TORA and ASDA. The reduction of these distances however differs for TODA and TORA on one side and for ASDA on the other. The TODA and TORA adjustments are based on initial distance of the main gear from the beginning of the runway since the screen height is measured from the main gear. On the other hand, adjustment of the ASDA is established upon a distance from the beginning of the runway to the nose gear.

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Figure 2: Line-up distance correction [Source: Airbus, Getting to grips of aircraft performance]

2.2.2 Take-off Distance

The Take-off Distance (TOD) of a specific aircraft on a dry runway (for definition see the subchapter Runway condition and contamination) is always greater of:

a. The horizontal distance along take-off path from the start of take-off to the point at which the airplane is 35 ft above the take-off surface assuming that critical engine fails at vef corresponding to v1;

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Figure 3: TOD one engine inoperative [Source: Jeppessen, JAA ATPL Training, 032 Performance]

b. 115% of the horizontal distance along the take-off path, with all engines operating, from the start of take-off to the point at which the aircraft is 35 ft above the take-off surface.

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Figure 4: TOD all engines operating [Source: Jeppessen, JAA ATPL Training, 032 Performance]

In case of a wet runway (for definition see the subchapter Runway condition and contamination), the take-off distance is greater of:

a. The take-off distance on a dry runway determined in accordance with the point b. at previous paragraph;
b. The horizontal distance along the take-off path from the start of take-off to the point at which the airplane is 15 ft above the take-off surface, consistent with the achievement of v2 before reaching 35 ft, assuming that critical engine fails at vef corresponding to v1.

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Figure 5: TOD one engine inoperative wet conditions [Source: Jeppessen, JAA ATPL Training, 032 Performance]

2.2.3 Take-off Run

At aerodromes with a runway not provided with clearway the Take-off Run (TOR) of the aircraft is equal to the TOD. If the take-off distance includes a clearway, the take-off run on a dry runway is a figure greater of:

a. The horizontal distance from the start of take-off to a point equidistant between the point at which vLOF is reached and the point at which the airplane is 35 ft above the take-off surface, assuming that the critical engine having failed at vef;

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Figure 6: TOR one engine inoperative [Source: Jeppessen, JAA ATPL Training, 032 Performance]

b. 115% of the distance with all engines operating from the start of take-off to a point equidistant between the point at which vLOF is reached and the point at which the airplane is 35 ft above the take-off surface.

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Figure 7: TOR all engines operating [Source: Jeppessen, JAA ATPL Training, 032 Performance]

In case of wet conditions the TOR of the aircraft is defined as greater of:

a. The horizontal distance along the take-off path from the start of take-off to the point at which the airplane is 15 ft above the take-off surface, achieved in a manner consistent with the achievement of v2 before reaching 35 ft;

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Figure 8: TOR one engine inoperative wet conditions [Source: Jeppessen, JAA ATPL Training, 032 Performance]

b. 115% of the horizontal distance along the take-off path, with all engines operating, from the start of take-off to a point equidistant between the point at which vLOF is reached and the point at which the airplane is 35 ft above the take-off surface (see figure 7).

Both definitions of the TOD and the TOR are specified in CS-25.113.

2.2.4 Accelerate-stop Distance

According to CS-25.109 the Accelerate-stop Distance (ASD) on a dry runway is defined as greater of the following distances:

a. The sum of the distances necessary to:

(i) Accelerate the aircraft from standing start with all engines operating to vef as for the take-off on a dry runway;
(ii) Allow the airplane to accelerate from vef to the highest speed reached during the rejected take-off, assuming the critical engine fails at vef and the pilot takes the first action to reject the take-off at the v1 for the take-off from a dry runway;
(iii) A distance equivalent to 2 seconds at the v1 for the take-off from adry runway;
(iv) Come to a full stop on a dry runway from the point reached at the end of acceleration in (iii) assuming that the pilot does not apply any means of retarding the aircraft until that point is reached.

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Figure 9: ASD one engine inoperative [Source: Jeppessen, JAA ATPL Training, 032 Performance]

b. The sum of the distances necessary to

(i) Accelerate the airplane from a standing start with all engines operating to the highest speed reached during the rejected take-off, assuming the pilot takes the first action to reject the take-off at the v1 for the take-off from a dry runway;
(ii) Distance equivalent to 2 seconds at the v1 for the take-off from a dry runway;
(iii) With all engines still operating, come to a full stop on a dry runway from the speed reached as prescribed in (ii).

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Figure 10: ASD all engines operating [Source: Jeppessen, JAA ATPL Training, 032 Performance]

The accelerate stop distance on a wet runway is the greater of:

a. The accelerate-stop distance on a dry runway determined in accordance with previous paragraphs;
b. The accelerate-stop distance determined in accordance with previous paragraphs, except that the runway is wet and corresponding wet runway values of vef and v1 are used.

The values of the TOD, TOR and ASD are directly affected, along with other factors as slope and wind, by mass of the aircraft. The higher the mass is the longer distances of the TOD, TOR and ASD are required by the aircraft, and vice versa. Amount of load influences the aircraft mass and it is how the operator could directly affect the take-off distances required to ensure they would not exceed appropriate values of the TODA, TORA and ASDA at the departure aerodrome.

2.3 Climb limitations

According to the different phase of the take-off flight different climb gradients are required. The take-off path in CS-25.111 is defined as path extending from standing start to a point of the take-off at which the airplane is at height:

- of 1 500 ft above the take-off surface, or
- at which the transition from the take-off to en-route configuration is completed and the final take-off speed is reached,

whichever point is higher. Both definitions assume that aircraft is accelerated on the ground to vef, at which point the critical engine is made inoperative and remains inoperative for the rest of the take-off, v2 is reached before the aircraft is 35 ft above the take-off surface and the aircraft continues at this speed minimally until reaching an acceleration height. The take-off flight path is then considered to begin at 35 ft above the take-off surface at the end of the take-off distance.

The take-off flight path can be divided into four segments. Each of these segments is characterized by configuration, thrust setting, speed and requirements for climb gradient so they correspond to the most critical condition prevailing in the segment. The flight path must be based on aircraft’s performance without ground effect(7).

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Table 1: Take-off segments characteristics

Figure 11: Take-off path segments [Source: Airbus, Getting to grips of aircraft performance]

CS-25.111 states that the airplane must reach v2 before it is 35 ft above the take-off surface and must continue at a speed as close as practical to but not less than v2 until it is 400 ft above the take-off surface. Nevertheless, obstacle clearance during the third segment must be ensured at any moment. Therefore, the acceleration height is at least equal or greater than 400 feet. On the other hand, the acceleration height is limited by maximum time of use of maximum take-off thrust, which in case of an engine failure at the take-off is set to 10 minutes. As a result, the en-route configuration at the end of third segment must be achieved within 10 minutes after the take-off.

As mentioned previously, for each segment of the flight there is a minimum climb gradient required by the CS-25.121. These gradients are defined according to the number of engines fitted to the aircraft and they determine a maximum mass for the pressure altitude and ambient temperature at which the aircraft is able to fulfill them. The lowest of these masses determines value limiting MTOM. This is also the reason why in some publications the climb limitation is called WAT, which stands for weight, altitude and temperature. In following table the minimum climb gradient requirements are laid down for each take-off segment.

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Table 2: Minimum climb gradient requirements

2.4 Obstacle clearance

Most of the airports are not located in a flatland and they are surrounded by natural or human-made obstacles. Especially during the take-off and landing phase of a flight, when an aircraft is close to the ground, all these obstacles could be possible danger and must be taken into account prior to the take-off. For a safe avoiding of the obstacles, regulation states requirements for both vertical and horizontal plane.

2.4.1 Vertical plane

To ascertain vertical clearance of all relevant obstacles, there was a need to determine a minimum vertical margin between the aircraft and each obstacle. Because of this, gross take-off flight path and net take-off flight path were defined.

The gross take-off flight path is a take-off flight path actually flown by the aircraft from 35 ft above the take-off surface at the end of the take-off distance to the end of the take-off path at optimum conditions(7). This flight path ensures compliance with required climb limitations.

On the other hand, CS-25.115 states that net take-off flight path must be determined so that it represents the actual (gross) take-off flight path reduced at all segments of take-off phase by a gradient equal to:

- 0,8% for two-engined airplanes
- 0,9% for three-engined airplanes
- 1,0% for four-engined airplanes

Therefore, net gradient flight path is gross take-off flight path minus a mandatory reduction. This reduction was introduced due to possible errors at controlling the aircraft by flight crew or some unpredictable conditions during the take-off.

An operator shall ensure that the net take-off flight path from the 35 ft above runway surface to 1500 ft above the take-off surface or point at which the transition to en-route configuration is completed, whichever is higher, must clear all relevant obstacles by a vertical distance of at least 35 ft. In case that during any part of the net take-off flight path the airplane is banked by more than 15°, it must clear all obstacles by a vertical distance of at least 50 ft as stated in EU-OPS 1.495. Moreover, the AFM provides a climb gradient decrement depending on aircraft bank turn. The aircraft operator must consider and apply also appropriate amount of this decrement.

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Figure 12: Take-off flight path with obstacles [Source: Airbus, Getting to grips of aircraft performance]

Consequently, obstacles in third segment of the take-off flight path influence minimum acceleration height. This height must be at least 400 ft above take-off surface, but must ensure also the vertical clearance of 35 feet between the net flight path and the obstacles in case of straight take-off and 50 ft if more than 15° bank turn is applied.

2.4.2 Horizontal plane

The vertical margin, as described in previous subchapter, must be kept from all obstacles within surrounding area of the indented track of departure. In some publications this area is also referred as a take-off funnel. Responsibility for clearance of the net take-off flight path from all obstacles within this area lies upon operator. The contour of accountability area is defined in EU-OPS 1.495 and it slightly differs for aircraft with wing span less than 60 meters and those beyond this wing span:

- An aircraft with wingspan greater than 60 meters must strictly clear all obstacles by horizontal distance of at least 90 meters plus 0,125 x D, where D is the horizontal distance the aircraft has traveled from the end of the TODA or the end of the TOD if a turn is scheduled before the end of the TODA.
- For an aircraft with wingspan of less than 60 meters a horizontal obstacle clearance of half the aircraft wingspan plus 60 meters plus 0,125 x D may be used.

From above mentioned it could be observed, that accountability area for obstacle clearance is widening linearly with flown distance of the aircraft. Theoretically, this distance could spread to extensive size, which would be highly unfavorable. This is the reason why a maximum width of surrounding area around the indented departure track was limited.

For those cases where the intended flight path does not require a track change for more than 15°, maximum lateral distance within which it is necessary to consider obstacles is:

- 300 meters, if the pilot is able to maintain the required navigational accuracy through accountability area (defined in AMC-OPS 1.495); or
- 600 meters, for flights under all other conditions.
For those flight paths, were track change of more than 15° is required, an operator needs to consider those obstacles which have horizontal distance less than:
- 600 meters, if the pilot is able to maintain the required navigational accuracy through accountability area (defined in AMC-OPS 1.495); or
- 900 meters for flights under all other conditions.

At each segment of the take-off flight path an aircraft has to clear obstacles at the determined vertical margin. If relevant, the reductions of climb gradient due to bank turn need to be also applied. Taking all obstacles within defined accountability area into account, each of these segments could be influenced by obstacle imposing a minimum net climb gradient. Therefore, a minimum gross climb gradient, higher than one required by the regulation, need to be set. This could result in possible MTOM limitation.

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Figure 13: Departure sector (Track change ≤ 15°)

[Source: Airbus, Getting to grips of aircraft performance]

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Figure 14: Departure sector (Track change > 15°)

[Source: Airbus, Getting to grips of aircraft performance]

2.5 Meteorological elements

Meteorological conditions of a day could significantly influence the determination of the MTOM. Changing of these figures could therefore considerably affect the correct value of the MTOM on a daily or even hourly basis. To this group of affecting elements belong direction and velocity of wind, pressure and temperature of ambient air.

2.5.1 Wind

Considering an influence of a wind on aircraft performance during the take-off phase, it is necessary to calculate a proper value of a wind component along lengthwise axis of the runway resulting in either headwind or tailwind component.

A crosswind component does not affect the value of the MTOM or v-speeds. Nevertheless, to ensure safety of take-off procedures the maximum demonstrated value of cross wind component must be published in the AFM. On the other hand, a wind component along the runway axis affects the take-off ground speeds and consequently required runway distances. If resultant wind component is a headwind, these distances are reduced following possible increasing of the performance limited MTOM. In case of a tailwind it is vice versa.

EU-OPS 1.490 (c) states that an aircraft operator when determining the MTOM must take into account not more than 50% of the reported head-wind component or not less than 150% of the reported tailwind component, but these conditions usually form part of the performance tables or software and the operator has to just consider the actual wind component for the MTOM determination.

2.5.2 Pressure altitude

If setting the standard pressure on an aircraft altimeter (1013 hPa), the instrument will read what is known as a pressure altitude. This altimeter setting is used when flying above the transition altitude to comply with a system of flight levels. Moreover, performance curves and tables in the AFM are usually given for varying pressure altitudes rather than aerodrome elevation. This is simply because a change in pressure altitude includes also a change of static pressure and density of the ambient air, which could be different from day to day. In general, the higher the airport pressure altitude the lower static pressure. Because the density of air is proportional to its pressure, the density of ambient air at the aerodrome is proportionally lower too. The consequences of these factors are reduction of a lift for a given true airspeed, reduction of a power and propeller efficiency, if applicable.

The lift equation for a given weight illustrates that if the pressure altitude increases, the true airspeed must be increased to compensate for the lift reduction. Obviously it takes longer to attain the higher forward speed necessary to produce required lift and therefore runway lengths needed for take-off becomes longer. Similarly, a drop in ambient air pressure and density (airport pressure altitude) negatively influences the engine performance as the available take-off thrust is reduced. This reduction could lead to prolonging of the take-off distances and reducing of all take-off climb gradients. Meeting all required climb gradients and available runway distances due to the airport pressure altitude reduction of airframe or engine performance could then result in a limitation of the MTOM.

2.5.3 Temperature

Unlikely to pressure, the density of ambient air is inversely proportional to its temperature. This means if outside air temperature (OAT) raises, the air density decreases. As mentioned in previous subchapter, a drop in air density has consequence in the airframe and engine performance reduction, which must be compensated by increased true air speed. Therefore, the take-off distances required by the aircraft could be prolonged and take-off climb gradients could be impaired, all resulting in a possible MTOM reduction. This is the reason why the operator must take into account the ambient aerodrome temperature at the time of the take-off.

Decreasing ambient air temperature leads consequently to increasing aircraft performance. This is true until possible icing conditions occur. Presence of these conditions depends on several factors, but in general these conditions exist when the OAT is at or below 5°C and sufficient moisture in any form is present (for example clouds, fog, rain, snow, etc.). Reduced lift, increased drag, increased total mass, increased stalling speed and possible block of pressure heads resulting in incorrect reading of the altimeters and speed indicators are just some of ineligible effects of icing. Therefore, the take-off is prohibited when frost, snow or ice is adhering to the wings, control surfaces or propellers and in such case the aircraft need to be de-iced prior to the take-off. Moreover, supplementary data and procedures covering flights in icing conditions must be applied.

2.6 Runway slope

A slope of a runway is usually expressed in percentages, where positive figure is considered to be an upward slope and negative figure downward. A scope of certified slopes is stated in the AFM, but it is usually not smaller than ±2%. In case of required extension to higher limits for operation on a particular runway, additional certification tests are necessary.

From performance point of view, a runway slope influences aircraft acceleration and deceleration ability as well as its maximum brake absorption capability. The ability of the aircraft to stop within the ASDA limited by the brake capability is discussed later in this chapter.

In case of sloping surface, component of weight acts along the runway, influencing the acceleration force. Therefore, an upward slope degrades the aircraft’s acceleration ability and increases required take-off distance and take-off run. On the other hand, the stopping distance after rejected take-off is shortened. Depending on a limitation, the MTOM could be then improved or lowered. In case of downhill, it is vice versa, but in general a magnitude of the upward correction is greater than the downward one.

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Figure 15: Runway slope [Source: Jeppessen, JAA ATPL Training, 032 Performance]

2.7 Runway condition and contamination

As mentioned in previous, the aircraft operator is responsible to consider a runway state for the MTOM calculation. So far there were discussed different calculation methods of required runway distances for dry and wet runways, but in addition, runway contaminants could also considerably affect the MTOM calculation.

2.7.1 Definitions

In following there are definitions of different runway states as they are stated in EU-OPS 1.480.

Dry runway; a dry runway is one which is neither wet nor contaminated, and includes those paved runways which have been specially prepared with grooves or porous pavement and maintained to retain “effectively dry” braking action even when moisture is present.

Damp runway; a runway is considered damp when the surface is not dry, but when the moisture on it does not give it a shiny appearance.

Wet runway; a runway is considered wet when the runway surface is covered with water, or equivalent, with depth less than or equal to 3 mm, or when there is sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water.

Contaminated runway; a runway is considered to be contaminated when more than 25% of the runway surface area (whether in isolated areas or not) within the required length and width being used is covered by the following:

i. surface water more than 3 mm deep, or by slush, or loose snow, equivalent to more than 3 mm of water;
ii. snow which has been compressed into a solid mass which resists further compression and will hold together or break into lumps if picked up (compact snow); or
iii. ice, including wet ice (3).

To be able to categorized particular state of a runway it is also necessary to define different contaminants.

Standing water; water of a depth greater than 3mm.

Slush; partly melted snow or ice with a high water content, from which water can readily flow, with an assumed specific gravity of 0,85. Slush is normally a transient condition found only at temperatures close to 0°C.

Wet Snow; snow that will stick together when compressed, but will not readily allow water to flow from it when squeezed, with an assumed specific gravity of 0,5.

Dry Snow; fresh snow that can be blown, or, if compacted by hand, will fall apart upon release (also commonly referred to as loose snow), with an assumed specific gravity of 0,2. The assumption with respect to specific gravity is not applicable to snow which has been subjected to the natural ageing process.

Compacted Snow; snow which has been compressed into a solid mass such that the aircraft wheels, at representative operating pressures and loadings, will run on the surface without causing significant rutting.

Ice; water which has frozen on the runway surface, including the condition where compacted snow transitions to a polished ice surface (8).

2.7.2 Effect on aircraft performance

The contaminant type predicts an effect it has on the aircraft performance. According to the different kind of influence they are divided into hard and fluid. As the hard contaminants reduce only friction forces of the aircraft, the fluid contaminants cause also precipitation drag and aquaplaning. The following table illustrates the categorization of different contaminants.

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Table 3: Contaminants categorization

Reduced friction has always negative effect on the aircraft performance, the TOR is prolonged and in case of rejected take-off the deceleration rate is impaired. On the other hand, it is not true for precipitation drag. It is composed of a displacement drag, produced by the dislodgment of the contaminant fluid from the path of the tire, and a spray impingement drag, produced by the spray thrown up by the wheels onto the fuselage. This additional drag has negative effect for the TOR as acceleration rate is reduced, but following a rejected take-off it has positive effect by improving aircraft deceleration.

The aquaplaning is a phenomenon when water on the runway surface creates an intervening water film between the tire and the runway surface leading to breaking of their contact. Under these circumstances the coefficient of dynamic friction is virtually reduced to zero causing tire traction and the aircraft wheels’ braking capacity dropping to almost negligible values. Moreover, the aircraft wheel steering for directional control becomes ineffective.

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Figure 16: Aquaplaning phenomenon [Source: Airbus, Getting to grips of aircraft performance]

The aquaplaning speed, at which the friction forces start to be considerably reduced, depends on a tire pressure and a specific gravity of the contaminant. It is necessary to take into account a hydroplaning penalizing effect when operating on the contaminated runway.

CS 25.1591 clearly states that supplementary performance information furnished by the aircraft manufacturer for runways contaminated with standing water, slush, loose snow, compacted snow or ice must be contained in the AFM. The information may be established by calculation or by testing, but if the information is not supplied, the AFM must contain a statement prohibiting the operation for such contaminant. In addition to performance information, the AFM should also include recommended procedures associated with particular contamination.

When it comes to the TOD, TOR and ASD determination, they are calculated in similar manner as it is on a wet runway although modified by effects of the contaminant. Even more, the ASD calculation on non dry runway surface may take into account also reversers’ effect as long as it is available and controllable.

In subchapter Obstacle clearance is stated that the net take-off flight path begins 35 ft above the take-off surface at the end of the TOD and must be guaranteed the vertical margin of 35 ft from all relevant obstacles to the end of the final take-off segment. The screen height (height at the end of the TOD) on a wet or contaminated runway is but reduced to only 15 ft. Consequently, although the net take-off flight path starts at 35 ft, the gross (actual) flight path begins at 15 ft above the take-off surface at the end of TOD. Interpretative/explanatory material (IEM) to EU-OPS 1.495 states that when taking off on a wet or contaminated runway and an engine failure occurs at the point corresponding to the decision speed (v1) for a wet or contaminated runway, this implies that the airplane can initially be as much as 20 ft below the net take-off flight path and therefore may clear close-in obstacles by only 15 ft(9).

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Figure 17: Take-off flight path on a wet and contaminated runway [Source: Airbus, Getting to grips of aircraft performance]

Different requirements for wet and contaminated runways, especially reduced screen height and allowed reverse thrust for the ASD determination in contrast to dry runways, could possibly lead to obtaining lower figures of the TOD and ASD. Depending on limitation it could result in a higher available take-off mass on such runway. This is the reason why the regulation specifies that on a wet or contaminated runway, the MTOM must not exceed the mass, which is permitted for the take-off on a dry runway under the same conditions.

2.8 Tire speed limit

The tire manufacturer specifies a speed limitation called a maximum tire speed (vTIRE). The maximum tire speed is referred as a true ground speed, and so limiting indicated air speed (IAS) and consequent take-off mass varies with the pressure altitude, temperature, wind speed, flaps setting but not with runway slope (22). This speed must prevent from tire structure damage and it is usually verified to allow all weights and associated v-speeds. Considering the fact that this limitation is only relevant until an aircraft is still on ground, vLOF is the most critical value and vTIRE must be always higher for a safe aircraft operation.

2.9 Brake energy capacity

In case of aborted take-off before reaching the decision speed v1 there is a need to stop an aircraft within the accelerate-stop distance available safely. A proportion of the kinetic energy reached at the time of the decision is absorbed by brakes and it is dissipated into the heat. This aircraft kinetic energy at the decision point is proportional to the take-off mass and a square of the decision speed v1.

Brakes manufacturers have to specify a maximum brake absorption capacity and so for a given mass there is a limiting speed at which a safe stop could be made. This speed is stated as ground speed and corresponding IAS varies with pressure altitude, temperature and wind. In contrast to tire speed limit, the maximum brake energy speed depends also on a slope of the runway as a change in height involves also a change in potential energy. In CS-25.109 defines: A flight test demonstration of the maximum brake kinetic energy accelerate-stop distance must be conducted with not more than 10% of the allowable brake wear range remaining on each of the airplane wheel brakes(8). This means that for certification purposes, the absorption capacity must be in addition demonstrated with worn brakes.

The brake energy limit speed vMBE must be determined in compliance with certified maximum brake energy capacity and must be less than or equal to v1. For most aircraft, vMBE is only limiting in extreme adverse of an altitude, temperature, wind or runway slope and it is verified to allow all masses for usual conditions. If calculated v1 for a given TOM is higher, the AFM will give the amount of weight to be deducted for each knot that v1 exceeds vMBE until they are equal.

If the take-off is initiated after prolonged taxiing the brakes could be already at fairly high temperature and their ability to absorb further energy could be impaired. In such cases, the flight manual specifies a time to allow for the brakes to cool.

2.10 Aircraft configuration and systems setting

An aircraft performance always depends on its configuration. It is obvious that gear retraction immediately after reaching the screen height positively contributes to the climb gradients, but when it comes to various configuration settings of flaps and slats it may not be so apparent. While particular configuration gives better performance on a short take-off distance, another gives better climb gradient after the lift-off. Throughout the optimization process it is required to figure out such configuration, which provides with the highest MTOM.

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Figure 18: Optimum flap setting [Source: Jeppessen, JAA ATPL Training, 032 Performance]

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