MIAMI — Following last week’s crash of a brand-new Lion Air Boeing 737 MAX 8 in Indonesia, the plane’s manufacturer has issued an emergency Airworthiness Directive, warning all the aircraft’s operators of a potential instrument failure that could force the plane to fly into a steep dive.
“Boeing would like to call attention to an angle of attack (AoA) failure condition that can occur during manual flight only. This bulletin directs flight crews to existing procedures to address this condition,” published Boeing today.
The service bulletin includes technical details—and even instructions to the operating flight crew—regarding the potential fault and how should the airlines address it.
The bulletin warns operators that the airplane’s AoA sensor might produce faulty readings, causing the plane to fly into a deep dive.
Later on, however, the manufacturer published an AD which applies to 246 Boeing 737 MAX planes, of which 45 are located in the United States.
The AD mandates that current 737 MAX operators “revise the airplane flight manual to give the flight crew horizontal stabilizer trim procedures to follow under some conditions.”
Thankfully, however, all 737 MAX operators are not being asked to ground their planes. The AD only asks for a revision of the flight training, which would prepare pilots to deal with the same incident that brought down the Lion Air jet.
Faulty Angle of Attack Readings: Possible Cause of Crash
The faulty [AoA] readings, according to Boeing, can pitch the stabilizer trim down for up to 10 seconds at a time.
The bulletin adds that the faulty readings might occur only during manual flight conditions.
“In the event of erroneous AoA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds,” says Boeing.
“The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 seconds after the electric stabilizer trim switches are released.”
A Boeing 737-800 Captain who works for a major US airline and has logged more than 6,000 hours of flight on this equipment tells Airways that “we train extensively on unreliable airspeed abnormals. The key is recognition, after which through power settings and ground speeds, the jet is controllable.”
Boeing continues in the bulletin stating that “repetitive cycles of uncommanded nose down stabilizer [may] continue to occur unless the stabilizer trim system is deactivated (…) in accordance with the existing procedures in the Runaway Stabilizer NNC.”
The 737 Captain replies to Boeing’s statement saying that such a situation “isn’t any worse than a Runaway Stabilizer trim,” suggesting that, with proper training, the fault is manageable.
“However, we must take into account the added chaos of aural warnings that would accompany the erroneous speed-low display,” explains the Captain, which at low altitudes, could be overwhelming for any flight crews.
“It would be a tough one to sort out,” the Captain admits. “Plus, to have to manually adjust the stabilizer trim… it is a real setup for disaster, especially without experience and intense training,” he concludes.
Re-Visiting the Accident
The brand-new Lion Air Boeing 737 MAX 8 (PK-LQP • MSN 43000 / LN 7058) crashed shortly after takeoff from Jakarta, Indonesia.
Flight JT610 had just departed from Jakarta International Airport (CGK) with destination Pangkal Pinang (PGK).
About 13 minutes after departing, the plane crashed into the Java Sea. Judging by the debris that was spotted in the sea, and the state of the retrieved bodies, the plane might have crashed at tremendous speeds.
All 189 passengers onboard the flight were killed in the accident.
As Indonesian authorities extend the search for more victims of the plane crash, the second black box is still yet to be recovered. The first one’s data has been retrieved and successfully downloaded.
“We have extended the operation for three more days,” told to Reuters Muhammad Syaugi, the head Basarnas (National search and rescue agency).
“This operation has been running for 10 days and the results from combing the sea surface and the seabed are declining, therefore the resources of Basarnas should be sufficient,” Syaugi told a news conference.
So far, up to 186 body bags have been retrieved, with 44 victims having been identified by the forensic experts.
What Is The Trim System For?
A trim system is vital to the correct—and safe—operation during an aircraft’s stages of flight.
By definition, to trim an aircraft is to adjust the aerodynamic forces on the control surfaces, allowing the aircraft to keep the set attitude without further control input, and without increasing the pressure on the flight controls.
In bigger, heavier commercial jets, these systems are hydraulic or fly-by-wire powered. The force needed to keep an aircraft properly trimmed are directly proportional to the airspeed and weight.
As explained by the Smithsonian’s Air & Space, “once the airplane is stable, whether in
“The purpose of trimming is to free the pilot from having to exert a constant pressure on the controls.”
The Boeing 737 MAX, and most commercial aircraft, also have an angle of attack (AoA) indicator. This system offers a visual indication to the flight crew of the amount of lift the aircraft’s wing is generating at any given angle of bank and airspeed.
The readings furnished by the AoA system are critical for the proper operation of an aircraft, as it gives the actual safety margin where the
FAA Airworthiness Directives To Follow
The FAA in the United States has announced that an Airworthiness Directive (AD) on the 737 MAX planes is imminent.
“The FAA plans to mandate the Flight Crew Operations Manual Bulletin by issuing an Airworthiness Directive,” said the organization via a Tweet. “The FAA (…) will take further appropriate actions depending on the results of the investigation.”
Moreover, the FAA said that it “has alerted affected domestic carriers and foreign airworthiness authorities who oversee air carriers that use the 737 MAX.”
At present, there are more than 120 Boeing 737 MAX 8/9s soaring the skies, as shown by Flightradar24.
Should the FAA and other agencies issue an AD that requires the immediate grounding and repair of all 737 MAX planes flying, airlines may be facing severe disruptions.
Boeing’s Bulletin In Detail
The two-page document published by Boeing goes into detail about how the trim failure can occur.
It is mentioned that the trim failure can be stopped and reversed by using the electric stabilizer trim switches, but may restart five seconds after the electric stabilizer trim switches are released.
Scarily, Boeing mentioned the following:
“It is possible for the stabilizer to reach the nose down limit unless the system inputs are counteracted completely by pilot trim inputs and both STAB TRIM CUTOUT switches are moved to CUTOUT.”
Boeing also included what the erroneous angle of attacks can cause, whether it is some or all the following indications and effects:
- Continuous or intermittent stick shaker on the affected side only.
- Minimum speed bar (red and black) on the affected side only.
- Increasing nose down control forces.
- Inability to engage
- Automatic disengagement of autopilot.
- IAS DISAGREE alert.
- ALT DISAGREE alert.
- AOA DISAGREE alert (if the AOA indicator option is installed)
- FEEL DIFF PRESS light.
The manufacturer concluded with what pilots must do to counteract the fault, should it replicate in the future.
Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used after the STAB TRIM CUTOUT switches are moved to CUTOUT.
So far, the FAA has been the only organization to issue statements. The European agency, EASA, has yet to comment.
This is a developing story. Stay tuned for more updates.
WITH CONTRIBUTIONS FROM ENRIQUE PERRELLA AND JAMES FIELD