Monday, June 1, 2009
The 'House' of Aviation
Ever watch the TV series "House"? For those who never have, this is a program about an MD named Gregory House, who is chief diagnostician at a major metropolitan hospital. He and his staff follow a logical progression in every episode to arrive at a correct diagnosis of rare afflictions. The most interesting part is that they accomplish this task within an hour, including commercials, every time!
In every aviation organization there is an individual equivalent to House. This person may be an A&P mechanic, inspector or avionics technician, but is always the "go-to" guy whenever a particularly tough maintenance problem arises.
It could be an intermittent glitch or a recurring pilot write-up. It could also be the item that has caused the replacement of every major component in the system, some of them multiple times, to no avail until it gets to a "House" who can actually analyze the problem, then restore the system quickly and inexpensively. This is the product of training, experience and deductive logic.
I testified as an expert witness in a case where a DC-9 had a pressurization spike that resulted in a passenger losing her hearing ability. The plane was on approach to Fargo, N.D., and experienced a rapid loss of pressurization when the throttles were retarded and the wing anti-ice system had been turned on. The flight crew’s response was to advance the throttle settings slightly to increase the bleed air pressure and volume. This absolutely correct action restored pressurization, but their following action, not writing up the anomaly, was unfortunately typical of some airline flight crews and put the safety of later passenger flights in this aircraft in jeopardy. Landing at Fargo on a through flight, where the airline had no maintenance personnel or even "on-call" contract mechanics available, the pilots had an unwritten policy of "working around" snags if possible, without delaying their flights or deferring the item (which in this case was non-deferrable using their MEL).
This situation is ubiquitous in the airline industry and is something to which the FAA always turns a blind eye. It will continue until a major accident results from "shirt pocket maintenance" such as this. Then those who are charged with overseeing the situation will be "shocked and dismayed" that the practice exists.
As a result, no maintenance action was taken until five days later when the aircraft was again reducing power to flight idle with the wing anti-ice system on, on approach to Hartsfield Intl Airport (ATL) in Atlanta. Fortunately, no one was injured this time and the flight crew was honest in their logbook procedures. Also, fortunately for the airline, the item was assigned, due to the recurrent nature of the gripe, to the airline’s local "House".
The first thing he did to isolate the problem was perform a "pressure vessel decay check," a procedure that was spelled out in early troubleshooting guides, but eliminated when the airline upgraded their maintenance manuals to a digital format. However, this mechanic had retained some of the earlier troubleshooting hard copy documentation for his own reference. The procedure involves pressurizing the cabin to 7 psi, then shutting down the pressurization source. At that point, you time the loss of pressure to 6 psi. If it takes longer than 30 seconds, the leakage is within limits. If not, start looking for the leaks. In this case, the test failed in about 15 seconds. Repressurizing the cabin, he asked another mechanic to walk around the aircraft exterior to listen for signs of leakage. Within a minute, the other mechanic found a cabin over pressurization relief valve flapper fully open due to air flowing through the valve itself. (Note that the flapper is spring-loaded closed, so on the ground, it would not normally be visibly defective.) They cleaned the debris, a strip of RTV sealant, from under the valve, retested the system as okay and returned the aircraft to service.
This sealant had been last disturbed during a heavy maintenance check and had flown in the leaking mode for almost six months. The problem only manifested itself when the engines were brought to flight idle, at altitude, with the wing anti-ice on. During those six months, mechanics had replaced the pressurization controller, the outflow valve actuator, a couple of bleed valves and reseated a number of other components. Think of all the time and money that was wasted, as well as the exposure of the passengers to potential injury from pressure excursions. In this case, it cost the airline $500,000 in injury damages awarded to the now-deaf passenger.
All the repair actually took was about one man-hour of troubleshooting by the local "House" to cure the problem. One can only imagine the money saved if the airline had properly trained its maintenance staff in the art of diagnosing problems. I know the training costs would be far less than what it eventually cost them in court and in the wasted labor and parts costs used to "get it out of town."
Deductive reasoning is critical to troubleshooting, as it is to medical diagnosis. The skill sets are identical and in both cases, someone’s life may depend on it.