Metro III: Propulsion System Malfunction + Inappropriate Crew Response

Metro III: Propulsion System Malfunction + Inappropriate Crew Response (PSM+ICR)

The Transportation Board of Canada (TSB) recently issued a report into the fatal loss on 10 November 2013 of a Fairchild SA-227 Metro III at Red Lake, Ontario.  The aircraft, C-FFVN, was operated by Bearskin Airlines.  Both pilots and three of the five passengers were killed in the accident.  The accident featured what a 1998 AIA/AECMA study termed a Propulsion System Malfunction + Inappropriate Crew Response (PSM+ICR).

The Accident

According to the TSB:

The landing checklist was completed and, at 1827:06, the crew advised Kenora FSS that they were 5 nautical miles (nm) on final approach for Runway 26 at the Red Lake Airport. At 1828, at approximately 500 feet above ground level (agl) and approximately 1.4 nm from the runway, the crew noted an aircraft malfunction but did not immediately identify the nature of it. Maximum power was applied to one or both engines, and the landing gear was initially selected up and then re-selected down before it could fully retract.

The crew declared an emergency with Kenora FSS and unsuccessfully attempted to initiate a climb. Shortly afterwards, the aircraft veered and rolled to the left, descended, and struck trees with its left wing. The aircraft continued through the trees and struck a series of hydro lines that ran parallel to Ontario Highway 125…

The initial contact with the trees and hydro lines arrested the aircraft’s speed and descent rate, and attenuated the force of the impact with the edge of the roadway. The aircraft cartwheeled down a slope which further reduced the force of the impact to the occupants in the rear of the aircraft. When the aircraft came to rest, the fuselage was broken in half forward of the overwing emergency exits and the front half of the aircraft was on fire.

The 406 MHz emergency locator transmitter did not activate during the accident.

C-FFZN (Credit: Bearskin Airlines via TSB)

C-FFZN (Credit: Bearskin Airlines via TSB)

The Investigation

On examination of the engine, Honeywell, manufacturer of the TPE331-11U-612G engine, concluded that a first-stage turbine blade of the left hand engine failed because of high-cycle fatigue as a result of the following factors:

  • Substandard porosity of the turbine blade material which resulted in inadequate fatigue capability and the creation a favourable location for crack initiation.
  • A minor increase in the mean stress in the blade fir tree region due to blade platform contact.
  • Stator burn-through which resulted in an uneven vibration on the first-stage turbine wheel assembly and heat stress on the turbine blades.
Burnt Through Portion of the Stator That Created Abnormal Blade Loading (Credit: TSB)

Burnt Through Portion of the Stator That Created Abnormal Blade Loading (Credit: TSB)

The TSB agreed and also found was that:

  1. As a result of the blade failure, the left engine continued to operate but experienced a near-total loss of power at approximately 500 feet above ground level, on final approach to Runway 26 at the Red Lake Airport.
  2. The crew were unable to identify the nature of the engine malfunction, which prevented them from taking timely and appropriate action to control the aircraft.
  3. The nature of the engine malfunction resulted in the left propeller being at a very low blade angle, which, together with the landing configuration of the aircraft, resulted in the aircraft being in an increasingly high drag and asymmetric state. When the aircraft’s speed reduced below minimum control speed (Vmc), the crew lost control at an altitude from which a recovery was not possible.

In their analysis the TSB say:

The loss of power and drop in N1 speed to 98% would have commanded the left engine propeller governor to attempt to maintain a constant engine speed of 100% by reducing the propeller blade angle. As a result, the left engine and propeller went from a low thrust condition to a high drag condition, with the fining out of the propeller blades. The left engine negative torque sensing (NTS) system was likely not operating because the engine had not completely lost power and was developing torque greater than the −4% value required to activate it. With the landing gear extended and flaps at ½, the aircraft was in a high drag asymmetric state.

The SA227’s NTS system may not always activate in response to an engine failure. The nature of the engine failure and aircraft profile may affect whether or not NTS activation parameters are reached.

Because the exact nature of the engine malfunction was not identified, the crew did not follow the standard operating procedures (SOPs) prescribed action of calling out the associated emergency procedure, which required them to stop and feather the propeller of the affected engine. This may have resulted from a belief that the NTS system would always activate in the event of a power loss and that NTS activation alone would provide adequate anti-drag protection from a windmilling propeller. Feathering the failed engine’s propeller would have decreased the drag associated with it and likely would have allowed the crew to maintain control of the aircraft.

The TSB comment that:

If pilots believe that the negative torque sensing (NTS) system in the SA227 aircraft will activate in the event of any power loss or that NTS activation alone can provide adequate anti-drag protection in the event of an engine power loss, there is a risk that flight crews operating these aircraft types may not initiate the Engine Failures In Flight checklist in a timely manner.

If there is no requirement for a boroscope inspection of the TPE331-11U-612G’s internal engine components in conjunction with the 450-hour fuel nozzle inspection, there is an increased risk that premature internal engine damage will not be detected.

If there are discrepancies between the fuel nozzle testing procedures described in the TPE331-11U-612G maintenance manual and the corresponding fuel nozzle overhaul manual, there is a risk that unserviceable fuel nozzles may be evaluated as serviceable and re-installed on aircraft.

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