A turboprop airliner's automatic ignition systems performed as designed, successfully relighting the aircraft's engines after they separately flamed out in heavy rainfall when the aircraft was on descent to land at Canberra Airport, an ATSB investigation has found.
The Virgin Australia ATR72-212A (ATR72-600), registered VH-FVN, was operating a scheduled passenger flight from Sydney to Canberra on 13 December 2018. Due to thunderstorm activity, the pilots were cleared by air traffic control to divert left of their planned track and subsequently held to the south-east of Canberra before tracking south then west to fly around the weather and then tracking back to land.
Shortly after commencing the descent into Canberra, passing 11,000 feet in heavy rain, the aircraft's right engine flamed out*. The engine's automatic ignition system engaged and the engine relighted within five seconds without pilot input. Then, approximately one minute later, passing 10,000 feet, the left engine flamed out and it too automatically recovered within five seconds, again without pilot input.
The ATR72's automatic ignition system worked as designed, correctly detecting the loss of engine power, initiating ignition and successfully relighting the engines without pilot input.
The incident highlights that while engine flameouts are not common in modern turboprop aircraft, they are still possible, according to ATSB Director Transport Safety Dr Stuart Godley.
"The ATR72's automatic ignition system worked as designed, correctly detecting the loss of engine power, initiating ignition and successfully relighting the engines without pilot input," Dr Godley said.
After the second engine flameout, the captain selected engine ignition to 'manual' in order to provide continuous ignition in an attempt to prevent any further flameouts.
"However, the selection of manual ignition potentially reduces the effectiveness of flameout recoveries, and should only be used when directed by checklists or a minimum equipment lists," Dr Godley said.
The ATR72's Pratt & Whitney Canada PW127M engines have a high-energy ignition system which is automatically disengaged following engine start, but in the event of a flameout, will automatically deliver a spark rate of between five and six sparks per second for 25 seconds before reducing to once per second until the engine relights.
Selecting manual ignition would also deliver an initial spark rate of between five and six sparks per second for 25 seconds before reducing to once per second. Consequently, if manual ignition has been ON for more than 25 seconds at the time of a flameout, it would not provide the high initial spark rate of automatic ignition, potentially delaying the relight process.
"This investigation highlights that reliable and effective systems and procedures exist to protect and recover from flameouts and it is important that pilots follow manufacturer procedures," Dr Godley said.
The investigation noted that ATR's Flight Crew Operating Manual did not prohibit the use of manual ignition in situations other than where the Electronic Engine Control unit was malfunctioning, and had no explicit direction for ignition to remain set to automatic.
Since the incident, ATR has ensured all operators of the ATR72-600 are aware of the appropriate use of the manual ignition, and is also reviewing operational documentation to determine whether this requirement could be explicitly included.
* A flameout is an unintentional extinguishing of the flame in the engine. This may result from interruption of any of the requirements for sustaining combustion, being fuel, air and heat.
You can find here the investigation report AO-2018-081: Engine Flameouts on descent involving ATR72, VH-FVN, near Canberra Airport, ACT, on 13 December 2018
