T/O overruns and veer-offs on slippery runways with crosswinds
Engine and tire failures are primary causes of high-speed rejected takeoff accidents.
By Nihad Daidzic
ATP, CFI-IA, MEI, CFI-G, AGI, IGI
President, AAR Aerospace Consulting
For every 3000 takeoffs only 1 RTO occurs. Most are accomplished safely.
A particular takeoff hazard exists when operating on slippery runways with significant crosswind (XW). This scenario may be more hazardous than the analogous problem of landings on slippery runways with XW. (See "Avoiding veer-off accidents on contaminated runways," Pro Pilot, Sep 2009, pp 54–58 and "Avoiding overrun accidents on contaminated runways," Pro Pilot, Dec 2009, pp 104–107).
Accident statistics show that better pilot training is required to address various takeoff problems. Existing full flight simulators (FFSs) are still not capable of realistic simulation of takeoff and landing ground runs on contaminated runways, with or without XW.
According to the updated joint FAA/industry Takeoff Safety Training Aid, which is based on takeoff accident statistics for commercial airplanes (certified under FAA FAR 25 and EASA CS 25) from the early 1950s through 2003, 1 rejected takeoff (RTO) occurred for about every 3000 takeoffs. Currently, about 18 million takeoffs are completed per year, and 1 RTO accident/incident occurs for every 4.5 million takeoffs.
A large percentage of high-speed RTOs resulted in overruns or veer-offs. Takeoff aborts were frequently initiated after V1 speed was exceeded. About 1/3 of RTO overruns/veer-offs occurred on wet/contaminated runways. Engine failure was responsible for about 21% of RTOs, while tire failure was responsible for about 22%.
The main conclusion of the study was that about 82% of the 97 studied RTO overruns were actually avoidable. In some cases, however, pilots correctly aborted takeoff after V1 was exceeded, since the airplane was unsafe or unable to fly.
Action speed and field lengths
The past few decades have seen a lot of discussion about the proper name for V1 takeoff speed. The current definition is the most accurate so far. It states that V1 is the maximum airspeed by which the pilot must take the first action to stop the airplane, and is also the minimum takeoff airspeed at which a pilot may continue OEI takeoff. Takeoff action speed is defined only in relationship to engine failure in multiengine airplanes. It does not explicitly address any other takeoff event, eg, tire/ wheel failure or inappropriate takeoff configuration.
In the case of balanced field length (BFL), takeoff distance required (TODR) and accelerate-stop distance required (ASDR) are equal and result in field length limited takeoff weight (FLLTOW ≤ MSTOW) for given runway and atmospheric conditions. An equal length of stopway and clearway (TODA = ASDA) added to the runway will also result in BFL conditions while increasing FLLTOW.
The upper figure on p 55 shows a sketch of runway, clearway and stopway geometry. Clearways are typically longer than stopways, enabling an increase in FLLTOW at lower V1s. Some regulators allowed for full clearway credit on wet runways (≤ 3 mm standing water depth), which means that liftoff distance equals takeoff run required (TORR), and the entire runway is used to achieve liftoff speed (VLOF ≤ VTE).
Initial climb to regulatory screen height (SH) of 15 ft (TODR), to avoid a severe TOW penalty of 35 ft SH on a wet runway, is then over the clearway. If there is no clearway, TODR will be limiting and the OEI airplane must reach 15 ft by the end of a wet runway in a manner consistent with achieving V2 before reaching 35 ft above reference zero (RZ).
FARs do not allow any clearway credit for wet runways and, technically, FLLTOW is limited by a TODR which is equal to available runway length (TORA=TODA) minus the corrections for airplane runway alignment, eventual rolling takeoff, etc. Available and required takeoff distances (TODs) are illustrated in the lower figure on p 55.
A particular scenario of ASDR, TODR and TORR for dry and wet runways is presented in the upper figure on p 56. Different combinations of balanced-V1s and FLLTOW exist, and one can choose operational-V1 from a range of airspeeds when actual TOW ≤ FLLTOW. In the first approximation we assume that tire rolling friction does not change whether the runway is wet or dry.
Contamination (> 3 mm water equivalent depth), such as slush or wet snow, would be significant exceptions, giving higher rolling resistance during takeoff acceleration and subsequently also lower braking friction during stopping.
Basic runway elements. An infinite number of combinations exist regarding the length of each section. (Diagram not to scale.)
Both of these effects would result in longer ASDR and TODR (and TORR), reducing FLLTOW significantly. For given accelerate-stop (AS) accelerations, the dry-ASDR enables higher speeds to abandon takeoff (≤ VMBE) than wet runways, which results in lower (dotted) curve for wet-ASDR.
An intersection of the wet-ASDR and TODR (15 ft) results in higher FLLTOW at lower V1. Although it can be fully expected that balanced wet-V1 will be lower, the fact that we can increase FLLTOW when wet sounds ridiculous.
The reason is simply a loophole in the EASA/FAR regulations which allows reduced SH for wet runways. An extreme case of wet-runway takeoffs would be allowing for full clearway credit. The initial climb to 15 ft after liftoff (and subsequently 35 ft at V2) is then entirely over clearway.
In its FAR 25 Amendment 25-92 from Mar 20, 1998, FAA disagreed with such practice. The agency does not allow any clearway credit for wet runways. In FAA's opinion, the existing wet-runway credit of 15 ft SH is sufficient.
By using FLLTOWdry (instead of FLLTOWwet), a range of operational wet-V1 speeds and a screen height of about 20 ft are obtained, which also improves meager obstacle clearance. Turbofan airplanes would normally prefer the lowest V1 (> VMCG) due to the large excess of thrust, while turboprops would normally desire highest V1 in the range available due to superior deceleration performance and a lesser excess of thrust.
Four-engine turbojets are normally restricted by a factorized TOD (net TODR being 115% of gross AEO TODR), while twin-engine FAR 25 airplanes are normally restricted by the OEI gross TODR, which has no additional operational safety cushions since the occurrence probability is 10–6 or less. There is no need to reiterate the importance of calculating and "flying" correct V1 speeds.