Saturday, February 23, 2019

Atlas Air 767 Flight 5Y3591 Plunges from 6,000 Feet

Three Atlas Air crew members tragically perished when their 26 year-old Boeing 767 plunged from about 6,000 feet in about 10 seconds. The freighter had departed from Miami and was on approach to Houston (IAH). The crew made no radio call, and there was no warning or indication of difficulty prior to the pitch over.

REVISED 11:38pm 22 Feb 2019: new slant ground speed plot
While the sudden and steep 767 descent resembles the final seconds of Lion Air 737 flight JT610, there is nothing to link this event and Lion Air otherwise, and there is nothing to suggest any similarity.

The flight data recorder and the voice recorder should provide information to diagnose all conceivable scenarios. The recovery of the debris and rebuilding the airframe will provide insight into the nature of any structural failure or explosion.  NTSB and FBI will have investigations.  The severity of the impact into water and muck will draw out recovery in order to sift the area thoroughly.

While the flight crew made no known call, likely due to workload trying to recover control, the ADS-B data persisted until nearly the very end. That suggests the crash recorders were functioning through the event. 

It should be noted that Autonomous Distress Tracking (ADT) will trigger telemetry under a severe departure from the normal flight profile, like this event, in the future (2021+).

Critical failure modes exist in the stab trim controls. These mechanisms are inspected periodically. Nothing has been highlighted for concern, no adverse trends, that I can find.

The Boeing 767 elevator controls have redundancy upon redundancy. The captain and the first officer yokes each have cables that connect to the left and right elevators, each driving a set of three hydraulic actuators. The actuators are powered by three independent (L,C,R) hydraulic systems, and the left and right side are fully redundant with cross-controls.  

The stabilizer is controlled by electric switches to drive hydraulic actuators. Stab trim is run on two independent hydraulics (L, C). It is possible to use hydraulic R to drive the stab if L and C are failed, and if fluid is still present in the Left plumbing.

Cutout switches for stab trim turn off the Hydraulic L and C pressure.  The stab trim uses brakes as well as actuators, where the brake holds the stab in position - released only when it is moving. There is no single point of failure in the control system. 

There are column position overrides to oppose stab mistrim. There is an additional stab trim control that operates as a supervisor to the yoke switch controls.

The autopilot can trim stab when in command only. Any manual stab trim or elevator input will disconnect a single channel autopilot.

The sudden and continuing dive is difficult to correlate to a stab trim runaway, as the pilots would have had time to stop the runaway and oppose the descent with elevator.

Cargo airplanes have been known to suffer from shifting cargo.  The most typical scenario is cargo sliding back in takeoff rotation, causing unrecoverable pitch up. There is nothing preceding the event to reveal a cargo shift. All that can be acknowledged is a cargo shift can cause an unrecoverable pitch down.

Structural failure, including in the stabilizer or elevator controls, can create an unrecoverable pitch down. An explosion could rupture structure and cause loss of control.  The debris field appears to be concentrated. Airplane break-up usually results in debris spread apart a wide area. It is too early to know where all the parts came down.

Pilot incapacitation leaves the airplane out of control. There is no preceding event to suggest pilot difficulties. The airplane was below 10,000 feet, so both pilot and first officer would be on station.

Weather does not appear to be a factor, except perhaps if turbulence contributed to dislodging cargo. 

Bird strike is not likely to cause a catastrophe of this nature. Loss of engine power does not lead to a sudden out of control dive. Birds taking out flight controls to the point the airplane cannot be flown is inconceivable. As well, it is inconceivable that birds could penetrate the flight deck and take out both crew members.  However, bird strike could have caused an upset or reaction which could have triggered something else.

The Boeing 767 is a very safe airplane.

There has been only one fatal crash due to airplane failure. Lauda Air 767 lost control when a thrust reverser deployed inflight. Two other 767 were lost due to failures, both occurred without injury, one a freighter burned up on the ground, the other the gear would not go down for landing.

Terrorist activities have taken the largest number of airplanes, six. Two of these were used to take down the World Trade Center, 9/11, which led to 2,657 lives lost - the most deadly of aviation incidents. 

Human error has taken down four airplanes, with only one fatal. The first 767 incident is known fondly as the Gimli Glider, where the airplane ran out of fuel due to a mixup of pounds Vs. kilograms and was successfully landed with only minimal damage.  Another 767 lost power just a month later when the engines stalled due to inadequate anti-ice at idle power  in descent, but they were successful in restarting and landed without further incident.

Engine failures (uncontained) have destroyed four 767s, all while on the ground, and none with loss of life. 

A 767 could not trim the stab briefly on approach, back in 1999. The system was reset inflight and landed without event. Nothing more is known for this one-time report.

The NTSB and FBI will brief the public as events unfold. A preliminary report will be forthcoming in less than 30 days. The official investigation will take one to two years to complete.  As a community we will find out what happened and respond to the vulnerability. For now, we mourn the lost of three souls, and heartfelt sorrow for their families, associates, and friends.

Crude plot of difference-based ground speed to account for the steep descent angle shows a clear buildup of speed as would be anticipated. It also shows what appears to be a brief low speed upset at the beginning of the descent. Whether the low-speed upset is real or an artifact will await the NTSB preliminary report.

Stay tuned!

Peter Lemme

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Peter Lemme has been a leader in avionics engineering for 38 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chaired the SAE-ITC AEEC Ku/Ka-band satcom subcommittee for more than ten years, developing ARINC 791 and 792 characteristics, and continues as a member. He contributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing Project Paper 848, standard for Media Independent Secure Offboard Network.

Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).

An instrument-rated private pilot, single engine land and sea, Peter has enjoyed perspectives from both operating and designing airplanes. Hundreds of hours of flight test analysis and thousands of hours in simulators have given him an appreciation for the many aspects that drive aviation; whether tandem complexity, policy, human, or technical; and the difficulties and challenges to achieving success.

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