From SKYbrary Wiki
Many aircraft accidents and serious incidents which have resulted in a Runway Excursion, especially in the landing case, have been partly attributable to a flight crew decision to use a runway which was not conducive to safe operations. Hindsight has indicated that a more careful assessment and tactical decision making process would have meant that a landing on that particular runway, at that particular time, would not have been attempted. The alternative might have been either the use of an alternative runway at the planned destination or a diversion.
Often, the context for a flawed decision on runway acceptance by flight crew, is a very strong motivation to try and recover a lost schedule or maintain a planned one. In addition, the increased focus by aircraft operators on minimising fuel burns can lead to a desire to support this goal, which can also occasionally interfere with balanced decision making and prompt the acceptance of an inappropriate runway, even when the aircraft is ahead of the planned schedule or where scheduled turn round times are perceived as challenging.
The historical record of runway excursion events shows that the runway conditions which are most often relevant, and which are often difficult for flight crew to assess, especially in the landing case are:
- Braking Action
- the likely braking action on a runway reported as ‘wet’ but not contaminated with water, or contaminated by snow and ice which is not hard packed and not uniformly distributed, is difficult to determine.
- the likely braking action on a runway contaminated with ice or snow cannot necessarily be assumed to be as last advised to ( and transmitted by) ATC.
- Cross Wind Component. The cross wind component when winds are strong, and therefore probably instantaneously quite variable in both speed and direction, and the extremes are in the vicinity of the Aircraft Flight Manual (AFM) crosswind limitations for the aircraft type.
- Low Level Turbulence. The possibility of variable low level turbulence which could affect the chances of achieving a landing within the designated touchdown zone.
The Role of the Flight Crew
A decision on whether an available runway should be used at all and, if there is a choice of runways, establishing which is the appropriate one to use, is part of any aircraft commander’s core duties. However, full advantage should be taken of all information sources and, where appropriate, of direct exchanges with ATC.
For the Take Off case, flight crew have a relatively easy decision to make and it should reflect the aircraft operator's SOPs and be based upon confidence that the relevant aircraft limitations in the Aircraft Flight Manual (AFM) and the Aircraft Performance assumptions can be met.
For the Landing case, the initial decision to select a runway, if a choice is available, and in any event to continue an approach to a landing on a chosen or otherwise designated runway, can be problematical. This is especially so if there are potential issues relating to the prevailing wind velocity, low level turbulence or runway braking action. If there is only a single runway available at the planned destination, the diversion option will need to be kept in mind and, if it is considered that there could be a ‘better’ destination runway option, ATC can be asked about this if they haven’t already taken the initiative.
The Role of ATC
ATC can assist flight crew in making appropriate decisions on whether to use an available runway. Proactively offering an alternative runway can sometimes prompt flight crew to consider their options more thoroughly than they might otherwise have done; this is especially relevant to approaching aircraft which do not have any opportunity to make direct observations of conditions until they are quite close to a landing. In some cases, especially for the landing case, assessment will lead to a decision to offer an alternative runway, whereas in others it may lead to a decision not to offer an alternative. The balance to be struck is between a focus on saving airborne time and fuel and a focus on supporting flight crew decision making from an operational safety perspective, with special reference to containing the risk of a runway excursion. There are three aspects worth considering before deciding whether, if the option exists, an alternative runway offer is appropriate:
- Wind Velocity
Whilst ATC cannot necessarily be aware of limiting crosswind and tailwind components for take off and landing of particular aircraft types, they will have a general appreciation which can be used, together with their access to continuous readouts and stored history of wind velocity readings, to aid their decision on whether to offer an alternative runway to the ‘default’ one for landing or for take off.
- Low Level Turbulence/Wind Shear
The ATC Tower controller will have a much better picture of current weather conditions relevant to the last few hundred feet and the landing, especially at times of rapid synoptic change, than any inbound flight crew can have. The controller will pass relevant information on directly observed conditions, and recent aircraft reports about weather conditions in the immediate vicinity of the airport; in addition, flight crew decision making on whether to complete an approach can sometimes be usefully aided by supplementing this with the proactive offer of an alternative runway which could be preferred, even if a delay would be involved. To a lesser extent, the same consideration also applies to departing aircraft.
- Runway Braking Action
The presence of, for example, packed snow and ice allow reliable tactical measurement of surface friction, which can be given to ATC by the airport authority for transmission to flight crew. However, other precipitation-related influences on braking action and directional control can be much more difficult for flight crew to consider, especially in rapidly changing conditions which can negate the value of pilot reports from preceding aircraft. Offering an alternative runway which is longer, apparently less affected by precipitation/slow drainage of water, or a potentially unhelpful crosswind component, may be extremely useful to aircraft on approach if surface water appears to be at risk of crossing the border between ‘wet’ and ‘flooded’. Since runway flooding is usually a temporary condition, a suggestion of delaying action may be welcomed.
- Runway Excursion
- Runway Surface Friction
- Accident and Serious Incident Reports:Runway Excursion
- Runway Overrun After Unstabilised Approach (OGHFA SE)
- Runway Overrun On Landing (OGHFA SE)