Safety Case Studies

Safety Case Studies examines aviation incidents and accidents that involved maintenance as either a probable cause or as a factor. The cases are culled from the National Transportation Safety Board database, from news reports and from foreign accident investigation bureaus.

A Gulfstream V Maintenance Saga

According to one message, the stuck landing gear was released by this method: "Shut down the right engine, pull T handle (isolating all hydraulics), then perform several positive G maneuvers until the gear locks down ... then fire up the right engine."

As this commentator observed, the four passengers on the Gulfstream G-5 jet no doubt enjoyed the ride! One of those passengers was Nike CEO William Perez. After the landing, he told reporters that one maneuver the pilots executed was a zero-gravity gambit where "you just sort of do a complete drop."

Track of the incident aircraft over Portland-Hillsboro Airport as the stuck main landing gear problem was evaluated.

Perez said he held up a magazine to see what would happen "and when we went down, the magazine stayed in the air - it was interesting."

The National Transportation Safety Board (NTSB) said the airplane bored holes through the sky for more than six hours while the problem was trouble-shot (see box).

Specifically, from the NTSB preliminary report:

"On November 21, 2005, at approximately 0600 Pac-ific standard time, a Gulfstream Aerospace G-5 ... was not damaged following a right main landing gear door sequencing failure near Portland- Hillsboro Airport (KHIO), Hillsboro, Oregon. The pilot, co-pilot, the cabin attendant, and four passengers were not injured. Nike Inc. was operating the flight with a destination of Toronto, Canada.

"The pilot said that they raised the landing gear on takeoff, [but] the right main landing gear (MLG) door did not retract. He said that the checklist instructed him to cycle the landing gear; when he cycled the landing gear, the right MLG became jammed in the half down position. "The pilot performed a low fly-by, and maintenance personnel on the ground photographed the bottom of the airplane. Gulfstream Aerospace engineers reviewed the photograph and advised the pilot on how to extend the landing gear. Six hours and ten minutes after takeoff, the airplane landed successfully.

"Post-incident examination of the right MLG revealed a black rubber skid mark on the inside of the MLG door; the skid mark terminated at a row of 27 bolts which hold a gap seal in place. The outboard tire of the right MLG truck exhibited seven evenly spaced gouges which matched the spacing of the gap seal bolts. The MLG door uplock linkage bungee actuator's ... exterior exhibited advanced exfoliating corrosion." [Emphasis added]

Other incidents involving bungee failure have occurred, and bungee replacement seems likely. In addition to the NTSB inquiry, a Federal Aviation Administration (FAA) official said his agency is investigating the incident as well.

Gulfstream issued a service letter to G-V, G500 and G550 operators on December 5 as a result of this incident, saying:

"It is suspected that corrosion caused the bungee to freeze in the extended position, contributing to the MLG door being out of sequence during the extension and retraction cycles ... With the door linkage system out of sequence, hydraulic pressure was attempting to close the door before the landing gear had completed its retraction cycle ... Gulfstream is evaluating the addition of a filler to the inboard gear door where the tire made contact."

Out of sequence actions caused by wheel-well obstruction, incorrectly closed doors and faulty microswitches continue to be the main reasons for failure to uplock, failure to extend and both genuine and false gear downlock warnings. Cycling can prove to be a poor solution.

Depending upon the individual system characteristics, often an emergency free-fall of the gear will prove to be the safest option, due to the sequencing being then "out of the equation." (NTSB report: www.ntsb.gov/ntsb/brief2.asp?ev_id=20051123X01879&ntsbno=SEA06IA019&akey=1)

Safety Pin Locks Out Thrust Reverser

A Japan Air Lines MD-90 landed at Kagoshima Airport on January 7 with the left engine's thrust reverser locked. The captain was able to stop the airplane on the dry runway without overrunning, and none of the 181 persons aboard was injured.

It was subsequently learned that mechanics forgot to remove a lock inserted into the left engine to prevent the engine thrust from being reversed after completing maintenance work the night before, according to airline officials.

A similar event occurred last July at Hokkaido. Following that incident, the airline instructed mechanics to insert the safety pin with a 24-inch long flag to prevent a recurrence, but this guidance was not always observed, chagrined airline officials confessed.