Why You Shouldn’t Run The Trim
I long ago lost track of the names and faces of the instructors who taught me how to properly trim an airplane, but their method was firmly imprinted. In that era of U.S. Air Force pilot training, if you left your thumb on the trim button for more than a second, you could get your hand slapped. Whether the control was electric, hydraulic or manual, you used it only in short, intermittent clicks, bursts or strokes. Your pressure on the control was light enough that you could sense the direction and magnitude of the trim change. You understood that if you held the trim switch, you ran the risk of over-trimming, with pitch forces becoming unbalanced in the opposite direction. Holding the trim switch was called “running the trim,” and it was a sin.
The accident description that follows can be a lesson for any pilot about running the trim.
On the night of March 19, 2016, a tired and frustrated pilot lost control of the Boeing 737-800 he was flying while attempting his second go-around in turbulent, gusty conditions. As the airplane accelerated and pitched up, he ran the trim--not just a little, but for 12 sec.--in the nose-down direction. The airplane went into an unrecoverable high-speed dive and struck the approach end of the runway on which he had intended to land. All 55 passengers and seven crewmembers died and the airplane was obliterated.
Why the pilot would run the trim like that and whether he was experiencing the same kind of somatogravic illusion as pilots had in similar go-around accidents was the subject of a lengthy investigation. The findings added some new perspectives to the conventional explanations of go-around loss-of-control accidents.
Note: For clarity and simplicity I will use conventional speeds, distances and heights in lieu of the metric units used on the actual flight.
FlyDubai Flight 981 was a scheduled international passenger flight from Dubai International Airport (OMDB) to Rostov-on-Don Airport, Russia (URRR). The crew reported for duty 1 hr. prior to the flight’s scheduled departure time of 2145 Dubai local time, and took off at 2237, 52 min. late. After cruising for 3 hr., 15 min. at FL 360, the crew requested and received the URRR current weather and landing runway information. Restated from metric units, it was wind, 250 deg. at 17 kt., gusting to 29 kt.; visibility, 3 sm; light rain showers; ceiling, 1,300 ft. broken, 3,000 ft. broken, 10,000 ft. overcast; temperature, 6C; dew point 3C; landing Runway 22. The braking action was good and there were temporary conditions of wind 250 deg. at 25 kt., gusting to 35 kt., and visibility as low as 3,300 ft., or a little over 0.5 mi. in rain and mist.
Twenty-one minutes later, ATC informed the crew there was a SIGMET in effect from the surface to FL 150 for severe turbulence. The SIGMET had not been included in the flight’s departure meteorological information. The crew’s discussion of this news was not recorded due to the length of the flight and the 2 hr., 14 min. duration of the cockpit voice recorder (CVR). However, only 5 min. later, at 0117 Rostov local time, they commenced their descent for landing.
When the crew checked in with Rostov Approach Control they acknowledged they had information Uniform, which included a notice of moderate wind shear. At 0136, Rostov Radar cleared the flight for the ILS Runway 22, and the crew then contacted Rostov Tower. Tower reported the winds were 240 deg. at 21 kt., gusting to 29 kt. and cleared them to land. When the crew captured the localizer and glideslope, the autopilot and autothrottle were engaged. The crew extended the gear, extended the flaps incrementally to 30 deg., and the airplane was fully configured by 0140. The crew set their approach speed and completed the landing checklist.
At the airplane gross weight of 117,000 lb., the landing reference speed (Vref) was 140 kt. The crew added 10 kt. to Vref for approach speed and set 150 in the mode control panel (MCP) target airspeed window. The Boeing 737 flight crew training manual (FCTM) procedure is to add half the steady state wind and all the gust up to a maximum of 20 kt. for approach speed. With the existing wind, the appropriate increment was 18 kt., not 10.
As the airplane descended through 2,600 ft., turbulence increased, as shown by 5 kt. airspeed deviations and vertical G loads of 0.8 to 1.25 G on the flight data recorder (FDR). At 1,850 ft., the captain, who was the pilot flying (PF), disengaged the autopilot and autothrottle. The airplane was equipped with a head-up display (HUD) on the captain’s side, and the captain is heard on the CVR referring to “flying the cue,” meaning he was using the HUD.
The captain had the runway in sight at 1,240 ft. QNH and about 4 mi. out. Twelve seconds later, at 1,100 ft., the windshear warning triggered. The aural warning was “GO-AROUND, WINDSHEAR AHEAD.” It was the predictive windshear feature based on the weather radar that triggered, not the reactive windshear feature, which is in the EGPWS and detects actual windshear. The reactive warning never triggered.
The captain immediately executed the windshear escape maneuver, which requires that gear and flaps remain extended and maximum thrust be applied. The airplane had a reduced thrust go-around feature as well as a full takeoff/go-around (TOGA) thrust setting. However, in addition to pressing the TOGA button, the captain manually advanced the thrust levers to the full forward position, resulting in an actual N1 rpm of 101-102%. Full TOGA thrust was 97% N1 rpm.
The captain followed the HUD guidance. Within 10 sec. the aircraft pitch was between 14 deg. and 15 deg. nose up. The go-around altitude set on the MCP was 2,300 ft., only 1,200 ft. higher than the altitude at which the go-around was initiated. The altitude acquire (ALT ACQ) mode activated 1,000 ft. before the MCP altitude, resulting in the need to pitch the airplane down significantly. As the captain applied nose-down force on the yoke, the first officer (FO) said “check the speed.” The captain, preoccupied with controlling the pitch, pressed the control wheel electric trim switches for about 4 sec. in the nose-down direction, while allowing the airplane to rapidly accelerate. The airplane continued to climb above the go-around altitude. At the airplane’s flaps 30 Vfe of 175 kt., the TE flap load relief valve automatically retracted the flaps to 25. The highest speed reached before the captain reduced power was 182 kt.. The windshear warning continued to be displayed.
During climb-out to FL 080 the captain told ATC they wanted to know the progress of another flight attempting to land at Rostov. That flight went around at 0153 due to windshear. The captain then decided to proceed to a holding pattern to wait out the wind conditions. To avoid icing they climbed to FL 150 and entered holding at the MN NDB southeast of the airport. They had sufficient fuel to hold for 2 hr., fly another approach, and divert to an alternate if necessary. Even after the hold, if they were able to land, they would have had sufficient duty time left to make the return flight to Dubai.
While in the hold, the captain is recorded as saying “I could see the airfield and we could go in, but…windshear warning, we went around.” He also asked the FO, “How was the go-around, was it a mess or was it OK?” He was aware they had exceeded the flap limit speed and told a flight attendant they would need to have a maintenance inspection.
The captain contacted FlyDubai via SATCOM, and they agreed with his plan to hold as long as possible and then, if necessary, divert to Mineralnye Vody Airport (URMM).
The captain left the cockpit at 0311 and the FO carried on a conversation with a flight attendant. He said “All the aircraft have left. We are the only one left here doing nonsense.” He continued, “But I do not think that with such weather, if it keeps being bad it is not worthy,” and “actually I don’t understand why they plan this type of flight to this Russian place at night when they already know during the daytime that there is a s**t of weather, they plan it at night?”
At 0320, the two pilots agreed to attempt another approach. The current weather as given to them by the tower was visibility, 3 mi.; ceiling, 2,000 ft.; wind, 230 deg. at 25 kt., gusting to 35 kt.; light rain showers and mist; severe turbulence and moderate windshear on final. The crew set an approach speed of 153 kt. (Vref 133 kt. plus a 20-kt. wind increment) based on their gross weight of 108,000 lb. They advised Rostov Approach that they would request a climb to FL 080 in the event of another go-around.
As the flight descended through 3,800 ft., turbulence began again. At 0333, a departing aircraft reported wind 260 deg. at 53 kt. and light icing as they passed 2,000 ft. Proceeding as before, the captain disengaged the autopilot and autothrottle and used the HUD to fly the approach. When the 1,000-ft. advisory sounded, the aircraft was fully configured, very close to localizer and glidepath, and 10 kt. faster than the planned approach speed. As the captain reduced the airspeed to 153 kt., a wind gust increased the speed to 168, and within 2 sec., to 176 kt. At that time, 0340:50, the captain said, and the FO repeated, “Go around.” There was no windshear warning.
Again, the captain pushed the TOGA button and pushed the thrust levers full forward. Unlike the first time, the FO prompted “Flaps 15” and the captain agreed. The crew also retracted the landing gear. The gross weight and drag were less than on the first go-around, but the thrust was still at maximum. As the airplane rapidly accelerated, the captain was unable to maintain the desired pitch attitude of 15 deg. The pitch attitude varied between 4 deg. and 18.5 deg., while the push forces on the control column varied up to 50 lb. over the next 40 sec.
At 0341:10 and again at 0341:18, the FO prompted the captain, “Keep it to 15 deg., nose up….” The aircraft exceeded the flaps 15 Vfe of 200 kt. and the flaps blew up to 10 deg. At 0341:30, with the aircraft at 3,350 ft., airspeed at 210 kt., pitch 10 deg. nose up, and with maximum thrust still set, the captain pressed the stabilizer trim switches to nose down. He held them for 12 sec. The aircraft began to pitch over and experienced significant negative G loading, up to -1.07 G, and the FO repeatedly called out “Be careful…no…don’t do that,” then “Pull it, pull it!” At 0341:49, the aircraft impacted the ground at a speed of about 340 kt., with 50-deg. nose-down pitch and in a left bank of 60 deg.