What happened
During flight in instrument meteorological conditions (IMC) at FL 290, the number one engine on a Honeywell ALF502R powered aircraft experienced a power rollback, dropping to flight idle. The area was characterized by significant thunderstorm activity and convective clouds with tops reaching FL 500.
As the crew descended to FL 260, the number two and number four engines also experienced power rollbacks. The loss of power persisted until the aircraft descended below 12,500 feet MSL, at which point power was restored to the three idling engines. The flight subsequently landed without further incident; there were no injuries.
The investigation
The investigation focused on the susceptibility of the unmodified Honeywell ALF502R engines to ice accumulation. It was determined that under specific temperature and moisture conditions, ice can accumulate on the supercharger exit guide vanes. This accumulation restricts the airflow to the engine core, leading to a reduction in power.
Findings
- The aircraft was equipped with four unmodified Honeywell ALF502R engines.
- Ice accumulation on the supercharger exit guide vanes caused the reduction in core engine airflow and subsequent loss of power.
- Previous airworthiness directives had prohibited flight in icing conditions above 26,000 feet when outside air temperatures were above -40 degrees C.