Asymmetric-thrust accidents continue to occur in normal operations and in training because of “a huge misunderstanding” about how best to retain control of multi-engine aircraft, said a Dutch former test pilot concerned that flight manuals do not reflect definitions of performance used in certification.
Pilots are often unaware that to maintain flight they need to have a margin above the published takeoff safety speed, which they might otherwise think is always sufficient to counter power loss from a wing-mounted engine, said retired Royal Netherlands Air Force Lt Col Harry Horlings.
A limitation in asymmetric operations with multi-engine aircraft–and published in the flight manual–is air minimum control speed, or Vmca. Problems can arise because pilots and engineers who determine Vmca are permitted to use a small bank angle, which permits control to be maintained at a lower speed than with wings level, according to Horlings, who spent 15 years in military experimental test flying.
He warned Flight Safety Foundation European aviation safety seminar delegates in Athens that FARs Parts 23 and 25, related European Aviation Safety Agency rules and other equivalent standards do not require the relevant bank angle used in certification to be published in flight manuals.
Horlings said many airline pilots “believe V2 is always safe, even after engine failure,” but “keeping their wings level can increase Vmca [by] 10 to 30 knots or more above the published value” derived with banked wings. To obtain optimum performance pilots would generally need to bank three to five degrees away from the inoperative engine; more roll could have a similarly detrimental effect on Vmca.
This means a bank angle more or less than that used to determine the “book” speed could increase the required engine-out velocity to higher than the calculated takeoff safety speed (V2), said Horlings. “The small bank angle is needed as soon as thrust asymmetry develops and as long as [it] exists to be able to control the airplane.”
‘A Sneaky Killer’
He said U.S. and European regulations currently “concentrate on loss of performance following engine loss, not on maintaining control. Accident investigation reports show there is a huge misunderstanding about this bank angle.”
Further, Horlings warned that not maintaining the certification engine-out bank angle during takeoff, go-around or the remainder of the flight–when a high thrust setting is required– might “turn a dead engine into a killing engine.”
Most Part 23 pilots consider that multi-engine airplanes remain controllable with asymmetric thrust at or above Vmca, which is indicated by a line on the airspeed indicator. But Part 25 pilots use V2, Horlings explained. “Therefore, pilots and accident investigators often incorrectly consider Vmca a useless speed.”
In many accidents, pilots have banked aircraft into the dead engine while airspeed was low and the operating-engine thrust setting was high, inevitably leading to a catastrophic accident. “This proves that test methods and conditions, as well as the configurations used during experimental flight testing to determine Vmca [were] never appropriately included as limitations or restrictions in flight or operating manuals, student textbooks, and not even in FAR and EASA [standards],” asserted Horlings.
In particular, problems can arise when manufacturers copy regulatory rules into their own manuals, because “not all regulatory text intended for certification purposes is applicable to airline or airplane operations,” said Horlings.
With no requirement to apply certification Vmca bank angles in manuals, he said most manufacturers say quoted values apply for a bank of not more than five degrees. “Not stating the required bank angle [necessary to validate] the listed Vmca might be the cause of many accidents. Vmca is a minimum speed to stay away from, just like the stall speed (Vs). Vmca really is a sneaky killer because it varies
a lot with bank angle, while Vs changes only slightly,” concluded Horlings.