While reading the 7Z9363 (a 2017 DC-9-80 [“MD-80”] very-high-speed rejected takeoff and runway overrun resulting from an undetectable wind-induced elevator jam that rendered the aircraft unable to rotate for takeoff) accident report, I noticed a curious and persistent omission (my emphasis):
...According to Boeing, in the history of this elevator design (which exists on all Boeing DC-9/MD-80 series and 717 model airplanes), this accident was the first notification that Boeing had received of an elevator jam occurring on an airplane exposed to ground gusts lower than 65 kts. Boeing noted that the elevator design first entered service in 1965 on the then-Douglas DC-9 airplane. [NTSB AAR-19/01, page 12 (page 28 of the PDF).]
In November 2018, Boeing provided an update that included copies of revised Fleet Team Digest (FTD) documents that it had distributed to operators... According to the FTDs, Boeing was developing a service bulletin (SB) for modifying the elevator structure of Boeing DC-9/MD-80 series and 717 model airplanes to attach a secondary travel stop that would prevent the excessive elevator TED travel that could result in the geared tab linkage becoming locked overcenter.
Boeing also reported that it was developing a revision to the AMM for Boeing DC-9/MD-80 series and 717 model airplanes to add new elevator wind damage inspection procedures, which would also include a lower wind speed threshold for the inspection. The inspection would involve verifying that an elevator was positioned trailing edge neutral or above. [Page 48 (64).]
The following analysis discusses the flight crew’s performance, including their preflight airplane inspection and control checks, preflight weather planning, and execution of the rejected takeoff after V1 (section 2.2), and evaluates the following:
- Mitigations to prevent takeoffs with an undetected jammed elevator for Boeing DC-9/MD-80 series and 717 model airplanes (section 2.4); [Page 50 (66).]
The NTSB notes that, given the circumstances of this accident and the potentially catastrophic outcome of an undetected jammed elevator, effective mitigations are needed. Therefore, the NTSB recommends that Boeing complete the development of a modification for Boeing DC-9/MD-80 series and 717 model airplanes that will prevent the possibility of elevator jamming due to ground wind exposure. [Page 58 (74).]
Thus, the NTSB concludes that, to ensure that the elevators of Boeing DC-9/MD-80 series and 717 model airplanes are inspected by maintenance personnel when exposed to ground gusts that meet or exceed criteria specified in the AMM, operators of these airplanes must maintain awareness at all times of the forecast and known wind where the airplanes are parked. [Page 59 (75).]
Missing from almost all of this discussion of DC-9 elevator jams is any mention of the MD-90, the largest DC-9 ever built, the last to be produced by McDonnell Douglas,1 and the penultimate DC-9 version overall. The report mentions the MD-90 only once, in connection with a 2001 Boeing operational bulletin (issued as a result of a 1999 incident) regarding wind-induced elevator jams:
Subsequently, on June 25, 2001, Boeing issued Flight Operations Bulletin (FOB) MD-80-01-02, “Flight Control Jam,” that alerted operators about the circumstances of the incident, including the belief that the wind forced the left elevator into a TED position beyond the design limits, causing it to jam.35
35The FOB, which applied to all Boeing DC-9, C9, MD-80, MD-90, and 717 airplanes, was also referenced by number DC-9-01-02, MD-90-01-02, and B-717-01-05. [AAR-19/01, pages 40-41 (56-57).]
This seems to indicate that wind-induced elevator jams were a concern for the MD-90 in 2001 (when the original bulletin regarding said jams was issued, applicable to all versions of the DC-9 - classic, -80, -90, and 717), but not in 2017-19, at the time of the 7Z9363 accident and aftermath thereof (all the discussion of said accident, and of the actions taken following same, mentions the classic DC-9, the -80, and the 717, but not the -90). Was the MD-90 given a different elevator system, or its preexisting elevator system modified, between 2001 and 2017, in a manner not true for any other version of the DC-9?
1: Assembly of the first prototype of the MD-95, the very last DC-9 version, started under McDonnell Douglas ownership, but McDonnell Douglas had merged with Boeing by the time it was rolled out bearing the 717-200 moniker.