Deceleration on the Runway
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The three primary methods of achieving deceleration on the runway during the landing roll or a rejected take off are engine power, brakes and mechanical spoilers. Effective and co-ordinated use of whichever of these are available will result in a stopping distance appropriate to the available runway ahead, or the desired runway exit point if sooner. The key to success is the properly co-ordinated use of all the available methods
The most important action to achieve deceleration from a speed at which the engines are still producing forward motion is to select all thrust/power levers to the ground idle position promptly, and if available, continue the action through to the selection of reverse thrust or reverse propeller pitch. This is the first action to begin deceleration and must especially be achieved without delay when a high speed rejected take off is initiated. Thrust reversers and reverse propeller pitch are most effective at high speeds. Selection at these relatively high speeds must be symmetrical because otherwise, directional control may be prejudiced. Once the aircraft groundspeed has reduced sufficiently, thrust/power levers should be returned to the ground idle position to prevent ingestion of any Foreign Object Debris which could be present, with reversers stowed once at taxi speed.
To be effective, braking action at any speed depends upon sufficient friction existing between the tyres and the runway surface achieved through freely rotating wheels.
Fundamental to this are:
- Achieving the maximum possible weight on braked landing gear assemblies
- The condition of the tyre tread
- The tyre inflation pressure
- The condition of the runway surface
Braking effectiveness will also be affected by the degree of brake wear, which must be within Aircraft Maintenance Manual specified limits, and the manually or automatically applied brake pressure as modified by system protections. These ensure adequate wheel rotation exists and is maintained before the commanded brake pressure is transmitted to the brake units and takes effect. When brake temperature indication is available on the flight deck, it must be within prescribed limits before a take off roll is commenced so that effective braking is available if a take off is rejected. System faults or inappropriate use of brakes during a long taxi out can raise brake temperatures into cautionary ranges where a delay for take off may be required.
Anti Skid Units are fitted to the braking systems of all modern transport aircraft. They modulate applied brake system hydraulic pressure before it is transmitted to the actuators in the brake units so as to obtain optimum braking based upon wheel rotational speed data received at the Unit. A minimum wheel rotational speed must be detected before any brake application will be achieved to prevent tyre destruction resulting from a locked wheel and to guard against the risk of Aquaplaning on wet or icy runway surfaces.
Autobrake Systems provide pre-selectable rates of deceleration which usually vary between 3 and 6 knots per second constant deceleration rate. Selection of ‘Low’ autobrake on an aircraft equipped with thrust reversers will usually have the effect of delaying brake application to allow the thrust reversers to work efficiently in reducing the initial high ground speed. Maximum manual braking through the toe brakes can produce deceleration rates of up to 10 kts18.52 km/h
per second subject to the operation of anti skid units.
Modern landing gear assemblies on fixed wing transport aircraft are fitted with carbon brakes, although steel brakes may still be encountered on older aircraft types. The application techniques for the two types differ slightly. Caution is required if a current aircraft type rating includes aircraft types or type variants which have both brake types and both types are likely to be encountered. If this applies to either pilot, the subject should be included in pre-departure and approach briefings.
Although the validation of tyre tread and inflation are matters for Line Maintenance in accordance with the applicable Maintenance Programme, pilot pre-flight external checks should include a positive assessment of apparent brake assembly and wheel/tyre status, including brake wear indicators. Any resulting uncertainties should then be referred to maintenance personnel. It is not possible to reliably assess whether the inflation pressure of each tyre on a multi-wheel landing gear is within prescribed limits merely by visual inspection. The record of tyre inflation checks and restoration of prescribed minimum pressures should be available to operating flight crew by reference to the Aircraft Technical Log
The mechanical deflection of parts of the wing upper surfaces and tail cone assembly can assist deceleration in two ways:
- Directly, by increasing aerodynamic drag on the moving aircraft. This can be achieved by raising upper wing surface panels called ground spoilers or by operation of a tail cone ‘clamshell’ type air brake. Both systems can also be used in the air on some aircraft types as in-flight air brakes, but the extent of their operation may be less in the airborne case than in the ground (weight on wheels) case. Extreme care may be needed if it is permissible for a particular aircraft type to carry out a rejected landing after initial touchdown, since not all ground settings of deployed spoilers and air brakes necessarily auto retract to the settings needed for a safe initial climb away.
- Indirectly by increasing the effective downward load on the landing gear and thereby increasing the efficiency of wheel braking.
Appropriate Use of Deceleration Devices
Although runway lighting, marking and signage may provide explicit indications of distance to go before the end of a runway, overrunning the end, either during a landing or a rejected take off, is not necessarily a consequence of an inability of the available systems to provide sufficient deceleration. Rather, decisions about whether to maximise the use of deceleration systems are sometimes flawed because a poorly-informed judgement is made about the ‘distance to go’. In the case of an abnormal landing roll or any rejected take off, the appropriate SOPs is to maximise deceleration rate using whatever methods are available, taking account of the degree to which built-in system protections against inadequate wheel rotation are present.
Runway Surface Conditions
The effectiveness of any attempt at deceleration from high speed on the runway after a landing or an Rejected Take Off decision will, of course, be affected by the surface friction at the time, with wet or contaminated runway conditions posing additional challenges. The borderline between ‘wet’ and water contaminated can be particularly difficult to determine for the landing case; flight crew often get little meaningful guidance from ATS because ATC themselves do not have water depth measurements and may only be permitted to offer 'unofficial observations' or pass on pilot reports made earlier. Also, when snow or ice contamination exists, different types of friction measuring devices measure different friction values when used on the same surface. None of the friction measuring devices are reliable on all types of contaminations. This adds another level of uncertainty to the runway surface condition.
Accidents and Incidents
Events where retardation methods were ineffective:
- A320, São Paulo Congonhas Brazil, 2007 (On 17 July 2007, the commander of a TAM Airlines Airbus A320 being operated with one thrust reverser locked out was unable to stop the aircraft leaving the landing runway at Congonhas at speed and it hit buildings and was destroyed by the impact and fire which followed killing all on board and others on the ground. The investigation attributed the accident to pilot failure to realise that the thrust lever of the engine with the locked out reverser was above idle, which by design then prevented both the deployment of ground spoilers and the activation of the pre-selected autobrake.)
- JS31, Kärdla Estonia, 2013 (On 28 October 2013 a BAe Jetstream 31 crew failed to release one of the propellers from its starting latch prior to setting take off power and the aircraft immediately veered sharply off the side of the runway without directional control until the power levers were returned to idle. The aircraft was then steered on the grass towards the nearby apron and stopped. The Investigation found that the pilots had habitually used "multiple unofficial procedures" to determine propeller status prior to take off and also noted that no attempt had been made to stop the aircraft using the brakes.)
- B734, Timbuktu Mali, 2017 (On 5 May 2017, a Boeing 737-400 made a visual approach to Timbuktu and slightly overran the end of the 2,170 metre-long runway into soft ground causing one of the engines to ingest significant quantities of damaging debris. The Investigation found that the landing had been made with a significantly greater than permitted tailwind component but that nevertheless had the maximum braking briefed been used, the unfactored landing distance required would have been well within that available. The preceding approach was found to have been comprehensively unstable throughout with no call for or intent to make a go around.)
- ATP, Vilhelmina Sweden, 2016 (On 6 April 2016, a BAe ATP partly left the side of the runway soon after touchdown, regaining it after 155 metres before completing its landing roll. It sustained damage rendering it unfit to continue flying but this was not noticed until five further flights had been made. Investigation attributed the excursion to lack of pilot response to unexpected beta range power and the continued flying to the aircraft Captain's failure to ensure proper event recording, accurate operator notification or a post-excursion engineering inspection of the aircraft. Systemic inadequacy in safety management and culture at the operator was identified.)
- B736, Montréal QC Canada, 2015 (On 5 June 2015, a Boeing 737-600 landed long on a wet runway at Montréal and the crew then misjudged their intentionally-delayed deceleration because of an instruction to clear the relatively long runway at its far end and were then unable to avoid an overrun. The Investigation concluded that use of available deceleration devices had been inappropriate and that deceleration as quickly as possible to normal taxi speed before maintaining this to the intended runway exit was a universally preferable strategy. It was concluded that viscous hydroplaning had probably reduced the effectiveness of maximum braking as the runway end approached.)
- Runway Surface Friction
- Rejected Take Off
- Landing Flare
- Surface Friction Measurement and Prediction in Winter Operations
- European Action Plan for the Prevention of Runway Excursions (EAPPRE) Edition 1.0, January 2013.
- ALAR Briefing Note 8.4 Braking Devices Flight Safety Foundation (2000)
- HindSight 12 - "Runway friction characteristics measurement and aircraft braking" by Werner Kleine-Beek
- An Investigation of the Influence of Aircraft Tire-Tread Wear on Wet-Runway Braking, T. Leland and G. Taylor, NASA, 1965