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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 FOD 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 AMM 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 <br />5.14 m/s <br /> 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 RTO 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.)
- DH8A, Rouyn-Noranda QC Canada, 2019 (On 23 January 2019, a Bombardier DHC8-100 failed to complete its intended night takeoff from Rouyn-Noranda after it had not been commenced on or correctly aligned parallel to the (obscured) centreline and the steadily increasing deviation had not been recognised until a runway excursion was imminent. The Investigation attributed this to the failure of the crew to pay sufficient attention to the external perspective provided by the clearly-visible runway edge lighting whilst also noting the Captain’s likely underestimation of the consequences of a significant flight deck authority gradient and a failure to fully follow relevant applicable operating procedures.)
- B744, Maastricht-Aachen Netherlands, 2017 (On 11 November 2017, a type-experienced Boeing 747-400ERF crew making a night rolling takeoff at Maastricht-Aachen lost aircraft directional control after an outer engine suddenly failed at low speed and a veer-off onto soft ground adjacent to the runway followed. The Investigation found that rather than immediately reject the takeoff when the engine failed, the crew had attempted to maintain directional control without thrust reduction to the point where an excursion became unavoidable. The effect of ‘startle’, the Captain’s use of a noise cancelling headset and poor alerting to the engine failure by the First Officer were considered contributory.)
- B739, Kathmandu Nepal, 2018 (On 19 April 2018, a Boeing 737-900 made a high speed rejected takeoff at Kathmandu in response to a configuration warning and overran the runway without serious consequences. The Investigation found that when a false Takeoff Configuration Warning caused by an out of adjustment switch had been annunciated just after V1, the Captain had decided to reject the takeoff because of concerns about the local terrain and locally adverse weather. It was noted that the aircraft operator did not provide criteria for rejecting takeoff up to or above the 80 knot crosscheck but that the Boeing reference QRH did so.)
- GLF4, Le Castellet France, 2012 (On 13 July 2012, a Gulfstream G-IV left the side of the runway at high speed during the landing roll at Le Castellet following a positioning flight after ineffective deceleration after the flight crew had forgotten to arm the ground spoilers. The Investigation found that pilot response to this situation had been followed by a loss of directional control, collision with obstructions and rapid onset of an intense fire. Contributory factors identified included poor procedural compliance by the pilots, their lack of training on a relevant new QRH procedure which Gulfstream had ineffectively communicated and ineffective FAA oversight of the operation.)
- 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.
- Global Action Plan for the Prevention of Runway Excursions (GAPPRE)
- 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