Guidance for Aircraft Operators
Advice in the Flight Manual or Operations Manual concerning landing weights and techniques on very slippery or heavily contaminated runways is there to enable the Commander to make a decision at dispatch and, when airborne, as to his or her best course of action.
At the flight planning stage the landing distance requirements at the destination and alternate aerodromes are satisfied taking into account the runway surface condition. It follows therefore that if the runways at the destination or alternate aerodromes are forecast to be contaminated then approved landing distance data, appropriate to the anticipated conditions, must be available in order to satisfy this requirement at dispatch.
When dispatching (pre-flight) to a contaminated runway the landing distance available at destination must be at least 115% of the approved contaminated actual landing distance and never less than the required landing distance for a wet runway.
Credit for reverse thrust may have been incorporated into approved performance data; however, the engine failure case should be considered for all flight phases.
Contaminated landing distance data is either published as advisory data in the Quick Reference Handbook (QRH) or theoretical certified (but not demonstrated) data in the AFM. EASA Certification Specification CS 25.1591 states that an aircraft may only be operated on contaminants for which certified performance data is provided in the AFM. Traditionally, contaminated performance data has been based on an analytical computation using aerodynamic and engine parameters demonstrated in flight test and an assumed wheel braking model for the runway effect. Therefore the contaminated runway data may not represent the performance that would be achieved, hence the additional safety margin applied at the dispatch stage to the certified (un factored) landing distance.
The operator is required to demonstrate that it can ensure safe operations, and by applying the same additional safety margin to the in-flight performance assessment as to the dispatch assessment demonstrates an equivalent level of safety. An FAA Safety Alert for Operators published in 2006 (SAFO 06012) recommends an additional safety margin of at least 15% on actual landing distance, except in an emergency.
Operators wishing to operate on contaminated runways may be required to demonstrate to the National Aviation Authority a level of safety for operations on contaminated runways. An acceptable means has been for the operator to apply company safety margins and additional required distances to the performance derived. Approved AFM contaminated performance data that has been derived from analysis and theoretical calculation of the demonstrated and certified dry runway Actual Landing Distance (ALD) has sometimes been found not to reflect the aircraft performance achieved. Research (as yet unpublished) is suggesting that a factor of up to 1.32 would more accurately reflect the performance the aircraft is likely to achieve, and a factor of up to 1.41 where reference information for operational corrections for runway slope and temperature is not available. These factors would obviously have an impact on the operator’s dispatch and in-flight assessment requirements. Operator analysis of the data from their Flight Data Monitoring (FDM) programmes, specifically looking at the piloting assumptions in the calculation of Operational Landing Data, may lead the operator to increasing the factor further.
Depths of water or slush, exceeding approximately 3 mm, over a considerable proportion of the length of the runway, can have an adverse effect on landing performance. Under such conditions hydroplaning is likely to occur with its attendant problems of reduced wheel braking and loss of directional control. Moreover, once hydroplaning is established it may, in certain circumstances, be maintained in much lower depths of water or slush. Crews should be familiar with the characteristics of hydroplaning, as its symptoms can be confused with a brake failure. A landing should only be attempted in these conditions if there is an adequate distance margin over and above the normal Landing Distance Required and when the crosswind component is within limits. The effect of hyroplaning on the landing roll is comparable with that of landing on an icy surface and guidance is contained in some Flight Manuals on the effect on the basic landing distance of such conditions.
Not only will reduced grip on a runway affect braking action, but also the ability to sustain high crosswind components too. Manufacturers may provide recommended crosswind guidelines for contaminated runways in their operational documentation. These guidelines are based on analytical computations and simulations, they assume uniform runway surface conditions, steady wind components, an evaluation of what the average line pilot can be expected to handle and a conservative assumption of an aft centre of gravity. The guidelines have not been demonstrated as part of the certification process. Operators should consider aligning their crosswind guidelines in accordance with their level of contaminated runway operational experience.
Operators should ensure that policies and procedures are in place to enable flight crew to assess whether sufficient landing distance is available at the time of arrival. In emergencies or abnormal configurations the flight crew needs to know the absolute landing distance (un factored Flight Manual distance) for the aeroplane configuration in order to evaluate whether to land immediately or to divert to another aerodrome.
Prior to commencing an approach to land a commander must be satisfied that the weather and condition of the runway does not prevent a safe approach, landing or missed approach having regard to the performance information in the Operations Manual.
Pilot reports of braking action (PIREP) may be broadcast and these will include the aircraft type and the time of the observation. However, these are unofficial, subjective reports and should be interpreted accordingly.
For runways contaminated with compacted snow or ice, reports of braking action are passed to pilots via the radio for each third of the runway to be used, for example, 'Estimated braking action for Runway 23 is: poor; medium poor; poor'.
Contaminated runway operations remain the exception rather than the norm in the UK, therefore additional guidance is required in order to help establish an equivalent level of safety. A stabilised approach is recommended using the maximum landing flap selection in order to minimise landing speed and landing distance. Research suggests that in airline operations the speed at the threshold is on average 5 kt to 7 kt above the scheduled value. This equates to a 10% increase in stopping distance or a 5% increase in the overall landing distance. Airbus currently quotes an 8% increase in ALD for an additional 5 kt approach speed. Floating above the runway before touchdown must be avoided as it may use a large portion of the available runway. The timely application of all retardation devices (speed brake, brakes and reverse thrust) should be applied after touchdown. Some manufacturers recommend the use of the highest available auto-brake setting allowed for landing as it avoids any in-built delays of the lower settings and ensures the prompt application of wheel brakes. When brakes are applied on a contaminated runway several skid cycles may occur before the anti-skid system establishes the right amount of brake pressure for the most effective braking. It is important to recognise that this is not a failure in the anti-skid system and that pumping the brakes or turning off the anti-skid system will only degrade the braking further. Braking should be maintained until the aircraft has reached a safe taxi speed, but crews should be aware that taxiways can be more slippery than the runway and that most anti-skid systems are no longer active at low speeds. If any doubt exists about the ability to perform a safe approach and landing a missed approach should be executed.
If a runway is found to be slippery during the landing roll the handling pilot should reduce speed to taxi speed before attempting to turn off the runway.