Although the National Transportation Safety Board has issued a probable cause for the fatal Mitsubishi MU-2 accident that occurred on Nov. 10, 2013, its report was not able to solve the mystery central to this accident: why the MU-2B-25’s left engine stopped running during the approach to Tulsa International Airport.
This accident is only the second fatal MU-2 loss since the FAA’s Special FAR 108 became effective in April 2008; the SFAR specifies mandatory initial, recurrent and requalification training for all MU-2 pilots and is considered a key factor in reducing the MU-2 accident rate.
According to the NTSB, the probable cause of the accident was: “The pilot’s loss of airplane control during a known one-engine-inoperative condition. The reasons for the loss of control and engine shutdown could not be determined because the airplane was not equipped with a crash-resistant recorder and post-accident examination and testing did not reveal evidence of any malfunction that would have precluded normal operation.”
The accident occurred on the pilot’s first solo flight in his recently purchased MU-2 on the way home from his initial training, on the same day that he took his final phase check of the training program.
The MU-2 pilot departed from Salina Regional Airport in Kansas at about 3:03 p.m. local time, on an instrument flight plan to Tulsa. After cruising at 17,000 feet, the pilot was vectored for a visual approach to Tulsa’s Runway 18L and eventually cleared to 6,000 feet then to 2,500 feet. About 10 miles from the airport, ATC cleared the pilot for a visual approach to 18L and told him to contact the tower. After checking in with the tower, the pilot was cleared to land on 18L and asked to reduce speed to 150 knots for a departing aircraft.
According to the NTSB, “After the airplane passed the Runway 18L outer marker, the airplane began a left turn. At 1544:48, when the airplane was about 90 degrees from the runway approach path, the tower controller transmitted, ‘Mitsubishi six Juliet Tango tower.’ The pilot replied, ‘I’ve got a control problem.’ The controller responded, ‘OK uh you can just maneuver there–if you can maneuver to the west and uh do you need assistance now?’ At 1545:06, the pilot replied, ‘I’ve got a left engine shutdown.’” No further transmissions were received from the MU-2 pilot.
Radar data showed that about 8 nm north of the 18L threshold, the MU-2 briefly leveled off at 2,200 feet msl–below the PAPI glidepath–then started turning back toward the runway. Descending through 1,400 feet msl, the MU-2 began turning left for almost 360 degrees, and it was during this turn that the pilot reported the control problem then shortly thereafter told ATC about the left engine shutdown.
The MU-2 crashed about 5 nm north of the runway centerline. According to the NTSB, “calculations indicated that shortly before the end of the radar data, the airplane’s lift coefficient (CL) reached the maximum CL (CLmax) for the flaps 20 configuration, which suggested that the final descent of the airplane into the ground followed an aerodynamic stall of the wing. This finding was consistent with the condition of the wreckage, its location very close to the last radar point and witness statements.” The MU-2 configuration when the accident happened was landing gear down and flaps 20.
In its study of the accident data, the NTSB stated that when the pilot made the last two transmissions to ATC, “the airplane was operating close to the 20-degrees flaps, one-engine inoperative minimum controllable airspeed (Vmc, 20) of 93 knots calibrated airspeed (kcas).” When the pilot leveled off at about 1,100 feet msl, the MU-2 was flying at about 95 kcas. “Associated with this level-off was an increase in required horsepower. The power increased to about the maximum available from one engine for the corresponding flight conditions…with full power on the operating engine, and at this speed, the airplane was close to the limit of controllability. During the final 360-degree left turn, the highest priority to ensure the safety of the flight would have been to increase the control margin by increasing the airspeed further above the 93-kcas Vmc, 20 speed. However, to increase the speed, a pilot would have to increase power on the operating engine (thereby exacerbating the thrust asymmetry and control problem at low speed, even if additional power were available); trade altitude for airspeed (which a pilot may be reluctant to do if the airplane is already at a low altitude); or perform some combination of these actions. A pilot could also increase the speed and margin from Vmc, 20, by retracting the landing gear, thereby lowering the airplane’s drag. Hence, at the time the power was increased between 1544:10 and 1544:30, the airplane was already in a difficult situation because of the combination of low altitude, low airspeed and the reported problem with the left engine.”
Engine Failure Investigation
The question about what caused the left engine to fail resulted from examination of the wreckage, much of which was burned in the post-crash fire. “The disassembly of both engines did not reveal any evidence of a pre-impact malfunction,” the NTSB noted. However, the left engine’s fuel shutoff valve (FSOV) was found in the closed position.
The FSOV can be closed two ways, with electrical power or mechanically by moving the condition lever to the emergency stop position. The FSOV is a two-position solenoid valve, and it is powered electrically to the open position or, using the Run-Crank-Stop (RCS) switches in the cockpit, to the closed position when the RCS switch is moved to stop.
According to the NTSB, the manual-close lever for the left engine’s FSOV “was in the open position. Subsequent testing of the left engine’s FSOV at three different facilities confirmed the valve operated normally.” Essentially, what this means is that the FSOV must have been closed electrically, not by movement of the condition lever, because the manual-close lever was not in the closed position.
There is only one way to move the FSOV electrically and that is using the RCS switch. And that is the central mystery of this accident: why was the FSOV moved electrically to shut off fuel flow to the left engine? The NTSB was unable to determine the actual RCS switch position because of the destruction of the cockpit. In MU-2 flight operations, the RCS switch is always supposed to remain in the run position, even when dealing with an engine failure.
There are two circumstances where MU-2 pilots would use the RCS switch to shut down an engine during flight: during training, this is how engines are shut down (and this pilot did experience four such shutdowns during his training) and for the in-flight Negative Torque Sensing system test, which is required after certain maintenance is done on the MU-2’s TPE331 engines.
NTSB Recommendations
As of the middle of last month the NTSB had published two recommendations regarding this accident, although there will be four in total. The first reiterates the Board’s May 6, 2013 Recommendation A-13-13, which wants the FAA to require installation of a flight data or cockpit voice recorder in all turbine-powered non-experimental-, non-restricted-category aircraft. The NTSB classifies this recommendation as “‘Open–Unacceptable Response’ because the FAA stated that it had not found any compelling evidence to require installation of cockpit recording systems as recommended,” according to the Board.
The second–Recommendation A14-99–has to do with checklists for the MU-2. One of the key actions resulting from the FAA Flight Standardization Board work that led to SFAR 108 was the requirement that all MU-2 pilots use the same FAA-accepted checklist published by Mitsubishi. Yet according to the NTSB, during his training “the accident pilot was provided a checklist that was not accepted by the [FAA] MU-2B Flight Standardization Board. The use of this checklist was not in compliance with the SFAR.”
The non-accepted checklist was identified as “For Training Purposes Only” and “generally followed the SFAR 108 accepted checklist content except for a few items that were in a different sequence,” the NTSB noted. “A fire-damaged copy of the unaccepted checklist was found in the cockpit wreckage and another copy was found in the aft portion of the fuselage. It is unknown whether the checklist was used during the accident flight.”
The NTSB interviewed the instructor who trained the accident pilot, and he said that the pilot used his training checklist during the first flight because it includes supplemental information that students find helpful. He said the accepted checklist was then used for all other training flights. The NTSB interviewed other MU-2 pilots trained by this instructor and also examined the cockpits of 10 other MU-2Bs to see which checklists they carried. “One former trainee reported never using the training checklist in flight and emphasized that only the FAA-accepted checklist was used. Another former trainee used the training checklist as the sole checklist for almost every flight.” There were non-accepted checklists in three of the 10 MU-2s that the NTSB examined, although some of the three also carried the accepted checklist.
This recommendation urges the FAA to issue written guidance to MU-2 instructors, owners, operators and pilots to use the FAA-accepted checklist. While the NTSB noted that there are differences between the accepted checklist and the training provider’s checklist, it did not point out any specific differences that could have been factors in this accident.
This article is based on the NTSB’s probable cause, narrative description and material placed in the official NTSB docket for this accident.