News Analysis
On March 21, 2022, a B737-800 aircraft of the China Eastern Airlines flight MU 5735 plunged into the ground killing all 132 on board. There were reports of rain in the area and overcast skies. Flightradar24 tracking of the Automatic Dependent Surveillance-Broadcast (ADS-B) taken from the Mode-S transponder on board indicated a sudden drop in speed coupled with loss of altitude. The resultant steep descent, arrested for a short period with increase in speed, followed by another steep drop in altitude and speed, before the impact, indicates a probable “loss of control”. Chinese authorities have grounded the B737-800 pending investigation and India’s Director General of Civil Aviation (DGCA) has announced increased surveillance of B737-800 aircraft operations. Is the aircraft system suspect?
Having flown the B737-800 for more than 8,000 hours, I can say with certainty that the aircraft is very safe, provided the pilots understand the system and flight characteristics, especially in adverse weather conditions. Speculations are rife on possible causes, including pilot’s suicide. A video of the final seconds of the crash before impact circulating in social media shows a near vertical descent of an aircraft without tail! The ADS-B data indicates a final descent rate close to 31,000 ft. per minute. This can be confirmed only with the Digital Flight Data Recorder (DFDR) reading, when it is located. The Cockpit Voice Recorder (CVR) will give a clear picture of conversations and audio warning sounds inside the cockpit. It is very critical to locate and decipher the DFDR and CVR recordings to identify the real cause.
Unlike the Silkair MI-185 crash on December 19, 1997 which descended at a similar high rate and killed all 104 on board, and which has been identified as pilot suicide by the U.S.’ National Transportation Safety Board (NTSB), the MU 5735 had the transponders on for signals to be recorded almost until the final moments. In the Silkair crash, the tail was intact and the clue was provided by the stabiliser screw jack, which was located in the wreckage. If it had been a mechanical system failure, the stabiliser setting would have been two degrees but it was found at the zero setting that was possible only with manual selection. The descent profile was possible only with manual control. MU 5735 had three pilots on board and the CVR read-out will clear any doubt on what transpired inside the cockpit.
The profile recorded by the ADS-B indicates the loss of control due to the flight being in the vicinity of a huge thunderstorm cloud. A satellite image of the area will indicate if CBs were present, like the fatal flight of AF 447 crash in the Atlantic on June 1, 2009. The track from Kunming to Guangzhou is hilly with ranges having peaks from 7,000 ft to 10,000 ft. If there are strong winds, eddies can exist up to two-and-a-half times the height of the hill. If cumulonimbus clouds tower over 40,000 ft, the core will have very strong updrafts which come out of the underside of the overhand, popularly known as the anvil, as extremely strong down drafts. The high descent profile of the crashed aircraft indicates a presence of the strong down draft.
The ADS-B data from Flightradar24 site has this crucial data in the 30 seconds between 06.20.43Z and 06.21.09. While the aircraft is maintaining the heading on 100 degrees, there is a sharp decline in speed from 457kts to 386kts and a descent rate building up to 31,000 ft per minute.
All modern jet aircraft engines have Full Authority Digital Electronic Control (FADEC) and the engine thrust is controlled by FADEC to prevent over-boosting and to have optimum thrust settings. The main setting is based on ambient temperature
The flight was maintaining 29,000 ft and the FADEC would have adjusted the thrust requirement for the ambient temperature at 29,000 ft. When the strong updraft moves the airmass from a lower level across the path of the flight, FADEC would sense the new warm airmass and reduce the thrust to prevent over-boosting of the engine. The only indication to the pilot, if he was monitoring the Total Air Temperature (TAT), indicating an increase in temperature. If the pilots had recognised that — unfortunately, very few monitor TAT — he should have taken the Altitude Hold Mode off and used control wheel steering to ride the waves. Leaving the autopilot engaged in Lateral Navigation and Vertical Navigation (LNAV/VNAV) mode will result in the aircraft maintaining altitude, autopilot will keep trimming the stabiliser back to relieve control forces, and speed will keep dropping. The reaction of the pilot to increase maximum thrust will have no effect as FADEC will not permit the engine to increase thrust due to the warm air mass encountered. The rapid drop in speed and the high rate of descent induced indicates a stall due to loss of control as the aircraft may have moved into areas with extreme downdraft. To un-stall the wing, the pilot may have used the stabiliser trim for nose down, which will result in the speed increasing and thrust increasing as the temperature at lower altitude will allow the thrust to increase. The resultant pitch up would give momentary gain in height followed by the pitch induced nose down due to the stabiliser trimming for nose down.
Were the crew aware of high altitude jet upset recovery manoeuvre? Were they trained for such an eventuality? Grounding a hundred B737-NGs in one airline while hundreds of others are flying in China shows more of a knee jerk reaction. The Indian DGCA following suit with surveillance makes it worse as far as safety consciousness is concerned. It shows a complete lack of knowledge by the Flight Standards Directorate of the DGCA, if they have advised this move. There are thousands of B737-NGs flying in the world and it is a safe aircraft. Panic reaction creates doubts in the minds of passengers. It is better to learn from accidents and take steps to prevent a recurrence. The DFDR and CVR of the crashed aircraft will provide the answers.
Capt. A. (Mohan) Ranganathan is a retired airline instructor pilot and aviation safety expert.
