Incident Description
The first officer of AirTran Airways flight 890, which preceded AirTran flight 867 in the accident airplane, identified and reported a leak from the right engine of a Boeing 737-200 during a postflight inspection at William B. Hartsfield Atlanta International Airport (ATL), Georgia. AirTran mechanics at ATL determined the leak originated from a chafed hydraulic pressure line to the right thrust reverser. They consulted the illustrated parts catalog (IPC), which is not designed as a troubleshooting document and lacks sufficient detail for such use. One mechanic contacted an AirTran maintenance controller in Orlando, Florida, for further instructions. The mechanics who initially identified the leak had limited experience with the Boeing 737, having previously worked for ValuJet Airlines, which operated only DC-9s, until the ValuJet and AirTran merger in September 1997.
Maintenance Actions
Based on the information provided by the mechanic, and without verifying the part number against the IPC or other appropriate maintenance documents, the maintenance controller instructed the mechanic to cap the leaking line and deactivate the right thrust reverser in accordance with AirTran's Minimum Equipment List procedures. However, instead of capping the hydraulic pressure line, the mechanics capped the right engine hydraulic pump case drain return line. The mechanics performed a leak check by starting the auxiliary power unit and turning on the electric hydraulic pumps to pressurize the airplane's hydraulic systems; no leaks were detected. A test requiring engine operation would have revealed the incorrect capping, but company procedures did not require such a test. Consequently, the overpressure and rupture occurred during the airplane's climb out, leading to depletion of system A hydraulic fluid.
Hydraulic System Failure
Depletion of system A hydraulic fluid activated the hydraulic low-pressure lights in the cockpit, alerting the flight crew to a hydraulic problem. The crew notified air traffic control of their intention to return to ATL and subsequently declared an emergency. The initial approach was high and fast due to the workload from performing AirTran's loss of hydraulic system A procedures and limited time; however, the crew configured and stabilized the airplane for landing.
Landing and Runway Excursion
Depletion of system A hydraulic fluid disabled the nosewheel steering, inboard flight spoilers, ground spoilers, and left and right inboard brakes. The flight crew landed using the left thrust reverser (the right thrust reverser was fully functional but intentionally deactivated), outboard brakes (powered by hydraulic system B), and rudder. The crew attempted directional control with the left thrust reverser and rudder, which depleted system A accumulator pressure that would have allowed three emergency brake applications. The right outboard brake was used without the right inboard brake at a higher-than-normal speed (Vref for 15-degree flaps is faster than Vref for normal landing flaps) and with heavy gross weight (the airplane had consumed only 4,650 pounds of the 28,500 pounds of fuel on board at takeoff). This consumed the remaining friction material on the right outboard brake, causing it to fail. The left outboard brake remained functional at that point. The lack of brake friction material on the right outboard brake caused one of its brake pistons to overtravel and unport its o-ring, allowing system B hydraulic fluid to leak and resulting in failure of the left outboard brake. Loss of all inboard and outboard brakes, nosewheel steering, and use of asymmetric thrust reverse caused the flight crew to lose control, and the airplane departed the left side of the runway, coming to rest in a ditch.
Probable Cause
The National Transportation Safety Board determines the probable cause(s) of this accident to be: (1) the capping of the incorrect hydraulic line by mechanics, which led to the failure of hydraulic system A; (2) the mechanics' lack of experience working with the Boeing 737 hydraulic system; and (3) the maintenance controller's failure to ascertain more information regarding the leaking hydraulic line before instructing the mechanics to cap the line and deactivate the right thrust reverser. Contributing to the cause of the accident were: (1) the asymmetric directional control resulting from the deactivation of the right thrust reverser; (2) the depletion of the left and right inboard brake accumulator pressure because of the flight crew's use of the rudder pedals with only the left thrust reverser to control the direction of the airplane down the runway; (3) the failure of the right outboard brake because the airplane was slowed without the use of the left and right inboard brakes and was traveling at a higher-than-normal speed and with heavy gross weight; (4) the failure of the right outboard brake after one of the right outboard pistons overtraveled and unported its o-ring, allowing system B hydraulic fluid to deplete and the left outboard brake to fail; and (5) the mechanics' improper use of the illustrated parts catalog for maintenance and troubleshooting and the maintenance controller's failure to use the appropriate documents for maintenance and troubleshooting.