Cautionary Tale — Continental Express Flight 2574

A cautionary tale of how imprecise language, along with its well-known accomplice latent safety culture, killed 14 people on September 11th, 1991.

Post is a follow up to Engineering Culture - Use Precise Language.

September 10, 1991

Overnight maintenance is scheduled to replace left and right de-ice boots on the horizontal stabilizer (part of the tail) of an Embraer EMB 120 Brasilia. The de-ice boots are inflatable bladders often found on leading edges of the wings and tail of small and medium-sized aircraft (large and military aircraft often use direct heating systems instead). The bladders can be inflated to break up ice formations.

Maintenance Begins

In most commercial airline maintenance situations, there is an impetus to turnaround aircraft before morning to re-enter revenue generating service.

• The de-ice boot maintenance is scheduled for 3rd shift. However, 2nd shift “has some time” and decides to help out, getting a head start.
• A 2nd shift mechanic begins work on right side, removing bottom screws on right side of horizontal stabilizer.
• The 2nd shift inspector offers to help — to further speed up the process. He begins work on right side of of the horizontal stabilizer removing top screws — then continues removing top screws on the left side.

22:00 : 2nd-to-3rd Shift Change During Active Maintenance

• 2nd shift mechanic does not complete shift paperwork or shift turnover paperwork — against company policy but condoned.
• 2nd shift inspector‘s paperwork states a single line “Helped mechanics remove the de-ice boots” (against airline maintenance management best practices by assisting in maintenance without additional inspector).
  • Perhaps the inspector believes the mechanic would provide additional detail in the mechanic’s report?
• 2nd shift supervisor believes work has only begun on right side, based on verbal discussion with 2nd shift mechanic. This right-side-only messaging is passed from 2nd to 3rd shift supervisor.

3rd Shift Maintenance Continues on Right Side

• 3rd shift supervisor learns from 3rd shift mechanic that process is taking too long on the right side.
• 3rd shift supervisor decides to limit maintenance scope to the right side given the limited time remaining. The left side maintenance will be performed during a future maintenance window (not uncommon).

Right De-ice Boot Maintenance Completes Successfully

• Embraer EMB 120 Brasilia aircraft re-enters service the morning of September 11, 1991.

Several Hours Later In Flight (September 11, 1991)

• The left leading edge on the horizontal stabilizer rips away.
• The lack of a leading edge on the left half of the horizontal stabilizer immediately puts the aircraft into a negative 5G pitchover (nose-down to point of inversion). The crew has almost zero time to react (and correct) before experiencing negative 5G (and likely passing out).
• Negative 5G exceeds the structural negative G limits of the airframe and results in the left wing detaching.
• Within a minute, fourteen people are dead.

It is later determined the left leading edge of the horizontal stabilizer is missing many screws on its top side. Screws removed by the 2nd shift inspector (against maintenance best practice and policy — inspectors inspect), not documented accurately in work logs — particularly relevant given 2nd-to-3rd shift turnover, and not caught by 3rd shift maintenance team completing the work.

Interesting Detective Notes

1. The left leading edge of the horizontal stabilizer is found very distanced from the rest of the wreckage which supports it being the first piece to detach from the airframe.
2. The screw holes (through which a screw would be secured) on the left leading edge of the horizontal stabilizer are completely uncompromised, with no signs of leading edge material tear-out around the screws. No tear-out clues investigators — the screws were missing.
   • Tear-out is a type of shear on a material being fastened. With tear-out, the fastener itself shears [through] the fastened material — if the screws were present, the detached leading edge would have tear-out markings similar to ripping paper from a stapled packet of papers.

NTSB Concludes

"The failure of Continental Express maintenance and inspection personnel to adhere to proper maintenance and quality assurance procedures for the airplane's horizontal stabilizer de-ice boots that led to the sudden in-flight loss of the partially secured left horizontal stabilizer leading edge and the immediate severe nose-down pitchover and breakup of the airplane. Contributing to the cause of the accident was the failure of the Continental Express management to ensure compliance with the approved maintenance procedures, and the failure of FAA surveillance to detect and verify compliance with approved procedures."

I Conclude

• The 2nd shift inspector violated policy by actively participating in maintenance procedures without a qualified inspector inspecting his own work.
• The 2nd shift inspector failed to precisely document the work he completed. Nowhere, written or verbally, was it conveyed to the next shift that the left side of the horizontal stabilizer had been touched.
• The 2nd shift mechanic did not complete shift paperwork or shift turnover paperwork.
• Continental Express management, at the time, accepted unacceptable safety management and safety culture.
• The 3rd shift maintenance team failed to realize many screws had been removed from the top left leading edge of the horizontal stabilizer.

There are many places where this accident could have been avoided. From an Engineering Culture — Use Precise Language perspective: if the 2nd shift inspector would have written “Removed screws from top right and top left leading edges of horizontal stabilizer," I postsuppose (not a word) the 3rd shift maintenance team would have found the positions with missing screws and securely reattached the left leading edge.

“Use precise language.”
— Competent engineers everywhere

Additional Reading

• Aviation Safety Entry