"The aircraft departed Stansted carrying the same fault with which it had arrived," the accident report concluded. In that sentence lies the tale of how one flight crew successfully coped with a faulty attitude director indicator (ADI) and how another flight crew failed to do so. The maintenance in between these two flights failed to identify and fix the root cause.
The damning details are contained in the investigation by the UK’s Air Accidents Investigation Branch (AAIB) into the Korean Air Lines B747-200F cargo jet’s brief Dec. 22, 1999, flight out of Stansted Airport near London. The flight ended in a fireball less than two miles from the airport. It was the outbound leg of a flight to Milan. The airplane, under another crew, had flown in from Tashkent, Uzbekistan.
During the inbound flight from Tashkent, the bank indication in the captain’s ADI froze and the comparator alarm sounded. The captain, who was the pilot flying (PF), noted that his ADI display differed from the first officer’s ADI and with the standby ADI. He correctly concluded that his display was erroneous and temporarily handed over control to the first officer.
A bit of explanation is in order. The captain’s ADI is connected to one of three inertial navigation units (INUs) installed on the aircraft, in this case INU No. 1. The first officer’s ADI is connected to INU No. 2.� Both pilots can select INU No. 3 as a backup source of attitude information. The standby ADI is fully independent, with its own gyroscope and battery power.
By switching from NORM to ALT, the attitude data flowing to the captain’s ADI can be switched from INU No. 1 to INU No. 3. When this was done on the flight out of Tashkent, the ADI displayed the correct roll information, and the warnings disappeared.
When switched back to NORM, the comparator alerts resumed. The captain set his ADI to ALT and resumed his role as pilot flying for the remainder of the flight to Stansted.
After landing at Stansted, the flight engineer noted the faulty ADI as "unreliable in roll" in the technical log. From the fault reporting manual (FRM), the flight engineer told investigators that he entered the appropriate codes, 34 41 AD 01. He also verbally passed details of the malfunction to the KAL ground engineer who met the flight at Stansted, to include how the display normalized when ALT was selected. The codes entered in the logbook referred to the captain’s ADI, and that it functioned correctly when switched to ALT.
Ground technicians could cross-reference the codes entered in the log to the fault isolation manual (FIM). This manual translates the FRM codes into maintenance action. Unfortunately, the technicians at Stansted did not have a copy of the FIM, according to the AAIB. If they had, they would have read that replacement of INU No. 1 was in order. A screwdriver never would have been applied to the ADI.
Two contract technicians (called "Engineer A" and "Engineer B" in the AAIB report) were on hand to resolve any squawk items before the plane’s next flight. They were part of the contract maintenance support provided to KAL at Stansted by FLS Aerospace Ltd.
Engineer A joined the KAL ground engineer in the cockpit and, under the latter’s observation, unscrewed the ADI from the instrument panel. A pin was found pushed back on the connector socket. At this point, since Engineer A was not a qualified avionics technician, Engineer B was called. On arrival, he pushed the pin into position with a satisfying "click." The test button was pushed as the ADI was in both the NORM and ALT position, with satisfactory results.
The technicians thought they’d fixed the glitch. Not so. The ADI self-test only showed that the instrument itself was functioning. It did not test the integrity of the data coming from the INUs.
How were three technicians seduced into believing they had fixed the problem, when the root fault in INU No. 1 still lurked? The AAIB declared bluntly that the KAL engineer "had insufficient technical knowledge of the ADI/INU interface." Engineer A was not an avionics technician. Engineer B was a qualified avionics technician, but he was helping to complete a process he had not started. There was no independent supervisory check of the work. Consider that omission to be the first of three critical fail points in this case.
INU No. 1 remained faulty. When the new crew for the flight to Milan boarded, the ADI was set to NORM, its input data coming from INU No. 1. The KAL ground engineer boarded to accompany the flight.
Various delays, such as taking nearly an hour to contact air traffic control on the incorrect frequency, had evidently frustrated the captain, who was the pilot flying. Of note, the 57-year old captain was considerably senior in time and experience to the 33-year old first officer.
Both the first officer and the 38-year old flight engineer may have been cowed by the captain’s ire. Based on the cockpit voice recorder (CVR), the captain twice admonished the first officer in a manner "construed by Korean listeners as derogatory," according to the AAIB report. On one occasion, the captain snapped, "Make sure you understand what ground control is saying, before you speak," and, "Answer them! They’re asking how long the delay will be."
Shortly after liftoff, the comparator warning sounded. The first officer said nothing and canceled the comparator warning. Consider this failure to inform the captain as critical fail point No. 2. The more experienced flight engineer said, "Bank is not working." Seconds later, he called out, "Bank, bank," suggesting that he was warning the captain of a bank angle greater than 30 degrees.
According to the AAIB, the first officer’s ADI and the standby ADI were correctly displaying the aircraft’s dire attitude. With the first officer inexperienced, overloaded, overawed and distracted, and the flight engineer being disregarded, the airplane’s fate rested entirely in the ability of one individual to get it all together. The captain flew his spurious ADI display all the way to the ground. The AAIB surmised that when his ADI failed, the captain "was apparently not able to recognize the problem." This was critical fail point No. 3.
The case prompted numerous changes at KAL and stands as an object lesson for operators in three key areas: basic flying skills in partial-panel situations, crew resource management, and maintenance troubleshooting by unqualified technicians with insufficient oversight. In short, inadequate airmanship was unable to compensate for inadequate maintenance.
The larger lesson is that redundancy is only as good as pilot training and the cockpit culture. Browbeating subordinates into silence or meekness is the human equivalent of flying with just one ADI–a denial of that great safety insurance policy known as redundancy. It applies to machines and to airmen.