FlarePath

What happened

NTSB summary, verbatim

The first officer indicated that he advanced the left engine throttle during the airplane's taxi in order to clear what he suspected was a stuck bleed air valve. Although he suspected that he had only moved the throttle "about an inch", FDR data revealed it had been advanced to and remained at an extremely high power setting for approximately 28 seconds. This action, which occurred without the knowledge of the captain or the tug operator and without application of the parking brakes, resulted in a substantial increase in thrust that overwhelmed the tug's resistance until the airplane impacted the tug.

Photographs

11the aircraft, and what the investigators found

The aircraft before the accident

Wreckage and site documentation · 10 photographs from the docket

Photographs are NTSB docket attachments — works of the US government, in the public domain. Captions are the Board’s own.

Probable cause

the Board's determination

The first officer's unannounced advancement of the left engine throttle, which led to a sustained application of high thrust that caused the airplane to impact the tug.

Occurrence sequence

1 stepsNTSB coding · CICTT taxonomy

1 · Pushback/towing
Ground collision
defining event

Findings

1 causal · 0 contributing

CAUSE
Personnel issues › Action/decision › Action — Copilot

Sequence of events

9 timed eventsfrom the FDR factual report · claude-fable-5-1 (in-session)

SRN 105390-105480groundp.1
Both engines at ground idle (N1 about 28/24%), then a brief symmetric increase to about 37/36% N1 with thrust levers near zero; pressure altitude about 136-142 ft throughout
SRN 105534-105562groundp.2
First reverse-range application: left TLA to -26 degrees with left N1 rising to 83.8% (fuel flow 8,560 lb/hr, EGT to about 462 C) while the right stayed at about -8 degrees and 38% N1
SRN 105574-105626groundp.2
Second application: both levers to about -16 to -33 degrees with left N1 reaching 93% and right 68%; left fuel flow 11,536 lb/hr, left EGT about 521 C, left EPR 2.016
SRN 105826-105874groundp.5
Third application: left to about -32 degrees (N1 92.8%, EPR 2.012) and right to about -18 degrees (N1 67.8%)
SRN 105888-105934groundp.6
Fourth application: left to about -33 degrees (N1 93.5%, EPR 2.023) and right to about -15 degrees (N1 54.8%)
SRN 106108-106200groundp.8
Series of short left-engine-only reverse applications (TLA about -31 to -35 degrees, N1 peaking about 70-72%) lasting 4-6 seconds each
SRN 106350-106360, 106436-106450, 106486-106496, 106510-106518groundp.10
Further short applications, mostly left engine, with N1 peaking 51-66%
SRN 106594-106612groundp.13
Final two-engine application: left to about -34 degrees (N1 87.5%, EPR 2.02) and right to about -16 degrees (N1 58.5%)
SRN 106714-106728shutdownp.14
Both engines shut down: N1 falling below 10%, fuel flow 32 lb/hr

NTSB analysis

from the final report

The first officer indicated that he advanced the left engine throttle during the airplane's taxi in order to clear what he suspected was a stuck bleed air valve. Although he suspected that he had only moved the throttle "about an inch", FDR data revealed it had been advanced to and remained at an extremely high power setting for approximately 28 seconds. This action, which occurred without the knowledge of the captain or the tug operator and without application of the parking brakes, resulted in a substantial increase in thrust that overwhelmed the tug's resistance until the airplane impacted the tug.

MCDONNELL DOUGLAS DC-9-83AMERICAN AIRLINES INC