Recently much correspondence has passed about our staff pilots, technical writers and several readers regarding our understanding — and sometimes misunderstanding — of engine-out performance of FAR Part 23 and SFAR 23 twins and appropriate pilot actions in the event of power loss.

This conversation was occasioned by the loss of several turboprop twins in accidents involving loss of control during deceleration to speeds near AFM Vmc. Some of that discussion will be addressed in a feature scheduled for our September issue. In the meantime, I thought it might be useful to share the story behind the loss of Bearskin Lake Air Service Flight 311 on Nov. 10, 2013.

Two pilots and three of the five passengers on board suffered fatal injuries when the Fairchild SA227-AC Metro III, C-FFZN, crashed just short of Runway 26 at Red Lake, Ontario, Airport after the left engine lost power.

A thorough investigation by Canada’s Transportation Safety Board (TSB) uncovered several factors that could have led to the accident including: possible crew confusion over the workings of the engines’ negative torque sensing (NTS) systems; the existence of overly complicated SOPs for response to engine failure while on approach; and the lack of manufacturer- and company-provided information on engine-out approach procedures. Most of what follows comes from the TSB’s report.

Bearskin 311 was a scheduled flight between Thunder Bay, Ontario, and Winnipeg, Manitoba, with stops in Sioux Lookout, Ontario, and Red Lake, Ontario. The segment from Thunder Bay and the subsequent departure from Sioux Lookout were uneventful.

At 1815, when inbound to Red Lake, the crew advised Kenora FSS of their position, estimated time of arrival at Red Lake Airport, and that they were still working with the Winnipeg Area Control Center (ACC) for air traffic control. The FSS specialist advised the crew as to the current wind speed and direction, and runway condition and asked them to report their intended runway for landing. The crew responded that they would be landing on Runway 26.

The 1800 METAR for Red Lake Airport was wind, 300 deg. (T) at 14 kt. gusting to 22 kt.; visibility, 10 sm in light snow and drifting snow; ceiling, overcast at 1,800 ft. AGL; temperature, -10C; dew point, -13C; overcast (specifically, cloud opacity at 8 oktas).

A special weather observation taken15 min. after the crash was wind, 320 deg. T at 10 kt.; visibility, 8 sm in light snow and drifting snow; scattered cloud at 2,000 ft. AGL; cloud opacity, 3 oktas (scattered). The wind observed by ground personnel at the airport at the time was similar to that reported in the 1800 METAR and was described as quite gusty.

At 1816, Winnipeg ACC cleared Bearskin 311 to Red Lake Airport for the VOR/DME Runway 26 approach and advised the pilots to contact Kenora FSS on 122.2 MHz.

The crew ran the descent checklist and, at 1817, advised Kenora that they had been cleared by Winnipeg for an approach. The pilots did not conduct a full approach briefing because they expected to encounter visual conditions prior to landing.

At 1827:06, after completing the landing checklist, the crew advised Kenora FSS that they were 5 nm out on final approach for Runway 26.

At 1828, when the Metro was about 500 ft. AGL and approximately 1.4 nm from the runway, the crew noted an aircraft malfunction but did not immediately identify its nature.

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. Moments later, the aircraft veered and rolled to the left, descended, and struck trees in a steep left-wing low attitude. The airplane continued through the trees and struck a series of power lines that ran parallel to Ontario Highway 125, before cartwheeling, breaking up and coming to rest in a wooded area adjacent to the highway. Fire broke out and consumed most of the aircraft.

The passenger in the left rear seat was able to open the left emergency over-wing exit and help extract a passenger who was jammed between the seats near the exit row. The other occupants did not survive the impact. One of the surviving passengers called 911 with a cell phone and local Ontario Provincial Police and fire and emergency services responded. The surviving passengers were transported to a local hospital.

Examination of the wreckage determined the flaps had been in the full-down position and the landing gear had been extended.

Investigation

The TSB determined that the captain was the pilot flying (PF) and was seated in the left pilot’s seat. He held a Canadian ATP endorsed for the Swearingen SW4 and SW5 aircraft types. He had been employed with the company since 2009 and had accumulated 5,150 hr. total with 3,550 in type. He had flown 251 hr. in the previous 90 days.

The first officer was the pilot monitoring (PM) and was seated in the right pilot’s seat. He held a Canadian commercial pilot license endorsed for SW4 and SW5 aircraft types. He had been employed with the company since July 2012 and had accumulated 2,200 hr. with 1,060 in type. He had flown 187 hr. in the previous 90 days.

The Metro III had been built in 1991 and had accumulated 35,475 hr. It was equipped with two Honeywell TPE331-11U engines rated at 1,000 hp each. Its maintenance checks and records were up to date and there were no outstanding discrepancies.

Using the FDR, CVR and ATC tapes, investigators quickly turned their attention to the engines. Ultimately they determined that the right engine and propeller had been functioning properly at the time of impact.

The left engine, however, suffered “greatly reduced power output to drive the propeller” due to a failure of the first-stage turbine blade that, in turn, caused damage to the remaining turbine blades. The cause of the initial failure involved metallurgical factors including fatigue and stator burn-through due to non-standard fuel distribution in the burner can.

The issue that puzzled the investigators was why the pilots lost control once the engine power began to roll back. Here is the TSB’s analysis of the pilots’ response to the engine-out situation:

A spectrum analysis of the CVR data indicated that the turbine blade failure was sudden and that there were no prior cockpit indications of an impending engine malfunction. The engine power loss was unexpected, and the crew had only 56 sec. between the time the left engine malfunctioned and the time the aircraft struck the trees. The crew did not verbally call out the emergency, likely due to difficulty in identifying the precise nature of the problem.

The following factors, said investigators, probably contributed to the crew’s difficulty in identifying the nature of the malfunction:

The right engine was at a low power setting when the left-engine power loss occurred, which would have made it difficult for the pilot flying to sense the yaw resulting from the malfunctioning engine.

The left engine continued to run, which resulted in engine readings of 98% engine rpm, with likely normal oil pressure, exhaust gas temperature and fuel flow. The low-torque indication in the cockpit would have provided some indication of the engine problem, but it was not noticed; and there was little time available to identify the nature of the malfunction.

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 one-half, 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. If pilots believe that the 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.

Because the crew did not identify the exact nature of the engine malfunction, they did not follow the SOP-prescribed action of calling out the associated emergency procedure, which required them to stop and feather the propeller of the affected engine. (Stop and feather is a knob that, when pulled, cuts off fuel flow and feathers the selected engine.)

This may have resulted from the pilots’ 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.

Analysis of CVR information indicated that the crew had configured the aircraft for landing and, when they experienced the engine malfunction, they initially retracted the gear as though they were on an engine-out approach. They then re-selected the landing gear down before it could fully retract. It could not be determined if the company’s non-precision engine-out procedure, which requires crews to reconfigure the aircraft several times over a short distance, may have influenced these actions.

At 1828:43, the crew reduced the power on the right engine to approximately 91% torque then made a further reduction to 54% torque (presumably to initiate a descent to the runway). FDR information indicated that the aircraft slowed to 101 KIAS and banked to the left after the second power reduction. Flight test data indicate that with the aircraft in a high-drag and asymmetric state, at this airspeed, the pilot flying would have had to input full aileron control deflection in an attempt to control the aircraft. Without further control input available, the pilot would have been unable to correct the aircraft’s rolling motion or recover from a stall.

At 1829:01, in response to the first officer’s instruction to climb, the right engine power was increased to 98% torque. This increase in power exacerbated the aircraft’s asymmetric state and resulted in the aircraft rolling left to 41 deg. of bank. The aircraft’s stall speed in this attitude, with full flap and the landing gear down, is approximately 98 KIAS. The aircraft’s speed slowed to very near the stall speed; therefore, the loss of control was likely the result of a wing stall.

There was insufficient altitude to recover.

In the end, the TSB determined “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. The 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.”

Rather than declaring a “probable cause,” the TSB publishes “Findings as to Risk” — basically lessons learned — and reports on safety actions taken. In this case the agency stated among its findings, “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.” 

In its safety actions following the investigation, Bearskin Airlines updated its SA227 single-engine-on-approach procedure memory items to include:

(1) Power — Increase to 60% torque or higher as required.

(2) Landing gear — Up.

(3) Flaps — One-quarter.

(4) Stop and feather (failed engine) — Pull.

(5) Landing gear (landing assured) — Down.

(6) Flaps — As required.

The company adds a note — especially important for the TPE331 operators. In all engine failure situations, the power lever should be advanced to MAXIMUM on the failed engine in order to minimize the drag from the windmilling propeller. Retarding the power lever on a failed engine will increase the drag on that engine and result in a control and performance penalty.

The airline also provided this information in its training manual that is certainly reasonable for all turboprop twin pilots to keep in mind:

Most aircraft AFMs have very little guidance for handling engine failures during the approach and landing phase of flight. The assumption is that this is covered with the Engine Failure in Flight checklist and the Single-Engine Landing checklist. Additionally, as pilots we have proceeded with the assumption that our NTS and autofeather systems will provide the necessary drag reduction.

The problem left for pilots is to determine what action to take at what distance from the runway. If the field is “made” it would be undesirable to initiate a missed approach when a successful landing could be accomplished. It is still critical, however, to have the aircraft in a configuration for a successful landing (prop feathered and gear down).

The farther you are from the runway, the higher and faster you will be and the more time you will have to accomplish a feathering. The closer you are, the less time you will have and it may be necessary to initiate a missed approach in order to feather the prop. This land/no-land decision will be dependent on the conditions of the day and the time that you have remaining prior to touchdown.

This is good food for thought and hangar talk. Be sure to see the accompanying sidebars discussing power loss recognition and asymmetrical thrust. B&CA

This article appears in the August 2015 issue of Business & Commercial Aviation with the title "Negative Torque Sensing vs. Autofeather."