While the two crashes of Boeing 737 Max 8 airliners have taken the vast majority of the world’s attention these past few weeks, for good reasons, a less followed mishap that claimed the lives of two Atlas Air pilots and a jumpseating pilot from another airline while the plane was on approach to George Bush Houston Intercontinental (IAH) has been generating a lot of discussion among professional pilots and armchair accident investigators who have noted two weird things about the crash.
First, as we noted in our initial coverage of the crash, it was an anomalous accident. The plane was on approach to Houston, and while there were thunderstorms in the area and pilots were asking controllers for and being granted deviations right and left of course to avoid the weather, the storms were not severe. Moreover, the way the plane crashed, steeply nose down but not otherwise out of control, with wings level, was an unusual crash signature, as well.
But the two most bizarre details behind the crash emerged when the NTSB reported on its findings. The plane was in an extreme nose-down condition (I chose my wording very deliberately…you’ll see why) and the power was set to maximum thrust.
In his ongoing analysis of the accident on his website, satcom.guru, former Boeing 767 avionics systems supervisor and DER Peter Lemme pointed out some interesting changes in the NTSB’s wording of an update on its ongoing investigation, in which investigators reported the following:
“Also, about this time, the FDR [flight data recorder] indicated that some small vertical accelerations consistent with the airplane entering turbulence. Shortly after, when the airplane’s indicated airspeed was steady about 230 knots, the engines increased to maximum thrust, and the airplane pitch increase to about 4° nose up and the then rapidly pitched nose down to about 49° degrees in response to column input. The stall warning (stick shaker) did not activate.
At some point subsequently, the NTSB changed that wording by deleting the phrase “to column input,” as shown with strikethrough characters, and substituting “nose-down elevator deflection.”
The report continued:
FDR, radar and ADS-B data indicated that the airplane entered a rapid descent on a heading of 270°, reaching an airspeed of about 430 knots. A security camera video captured the airplane in a steep, generally wings-level attitude until impact with the swamp. FDR data indicated that the airplane gradually pitched up to about 20 degrees nose down during the descent.
So what happened, in short, according to the NTSB details so far is as follows: The plane was level at 6,200 feet at 230 knots while it was on its way down to a cleared altitude of 3,000 feet when it gained 100 feet of altitude, to 6,300 feet, encountered what the investigators say is consistent with light turbulence and then, and this is where the data gets strange, its power increased to full thrust, its nose rose four degrees (consistent with the application of full thrust) and then its pitch went from four-degrees nose up to almost 50 degrees of nose down, with the elevator moving in support of that change from typical cruise attitude to a severe nose-down pitch.
The change in wording seems very possible to avoid giving the impression that the elevator deflection was commanded by one of the pilots by pushing the control column forward. The new wording seems intended to allow the possibility that the elevator deflection was the result of some kind of mechanical malfunction that has not been identified so far. The 767 was the subject of a series of airworthiness directives over the past 19 years and as recently as 2014 that called for the mandatory replacement of shear rivets in the elevator bell crank assembly. The AD noted that the failure of those rivets could lead to a hardover and a significant pitch upset, though that upset, the FAA opined, was recoverable by the crew. The AD should have been complied with by now. So far the NTSB has given no indication that faulty rivets in the elevator bell crank assembly, or elsewhere, might have been to blame.
Even given all that, the inexplicable thing, however, is the application of full thrust, which happened nearly simultaneously with the extreme pitch down elevator movement. The power was left at full thrust for the duration of the dive up until the plane impacted terrain.
Another important detail that the NTSB reported is that the plane went from almost 50 degrees of nose-down pitch to 20 degrees of nose-down pitch as it was in the dive. Investigators have not given an opinion on the cause of that change in pitch. While it is consistent with an attempted recovery from the dive, the application of max thrust is not.
Some observers, including professional pilots, have noted that the application of full power might have been a part of a recovery from a nose-low attitude, as adding power will raise the nose, though the sequence is problematic, as the power was introduced before the elevator moved the nose down. The recovery from a nose-down condition was not yet needed when power was applied, as the nose was level or in a modest climb attitude.
In response to Lemme’s post, the NTSB pointed out that it does not release its conclusions until an investigation is complete, and it stressed that any conclusions about about probable cause are premature and speculative. The same disclaimer clearly applies to our coverage too.
Below is video released by the Chambers County Sheriff’s Office showing the crash. Readers should be advised the video, though taken at a distance, shows the moment of impact.