Maintenance matters

Maintenance matters

Editor’s Note: The following accounts are from actual mishaps. They have been screened to prevent the release of privileged information.

It’s time to look at my favorite aircraft, the wonderful A-10 Warthog. Unfortunately, Hog Keepers have their bad days, and have damaged a few aircraft when they didn’t follow the procedures or pay attention to what was going on.

That Gap Is Normal

Three test cell personnel were performing a TF-34-100A engine run on a test stand in the hush house. During the run, a metal-covered test cable was ingested into the engine through a small gap between the engine inlet screen and the engine intake. External damage was visible to the engine fan blades only. After a complete teardown and inspection, several first stage fan blades were damaged and the outer shroud parts were nicked. No parts or pieces of the metal clad cord were ingested internally into the engine.

The true cause of this mishap is that the engine inlet screen does not remain tight against the front of an engine during maintenance runs. This small “gap” has been identified numerous times indicating a problem with the engine inlet screen. This gradually became accepted as “normal” operation instead of being identified as a problem with engine test stand. Tech data procedures were followed each time the inlet screen was installed. However, during each engine run the screen would be pushed away at high throttle settings and separate from the engine approximately 1-2 inches. The tech data does not state the need to continually ensure the screen stays against the engine. In addition to this “accepted practice,” the crew took a shortcut as the test set cable was not long enough to allow for correct and safe routing of the cable through securing clamps and tiedowns, so it was not restrained during the engine run.

Who is to blame here, the system or the people? In my opinion both are to blame. Supervision knew of the problem and didn’t fix it. The workers knew of the problem and accepted it. The team didn’t apply appropriate risk assessment to their daily tasks. What else is out there that is accepted but shouldn’t be?

TEMS Is Wrong, Not!

While in the local traffic pattern, a pilot initiated a planned go-around, at which time the No. 1 engine experienced a compressor stall and subsequent engine fire. The pilot accomplished appropriate critical action procedures and extinguished the fire. The pilot successfully completed a single-engine landing and egressed the mishap aircraft without injury.

Maintenance messed this one up. Engine technicians did not comply with tech data and classified actual turbine engine monitoring system (TEMS) hits for vibrations (code 36), which are grounding discrepancies, as nuisance hits a non-grounding discrepancy. In addition, the tech data the troops were using was missing a change which led the maintainers to falsely categorize the actual TEMS code 36 hit as a nuisance hit. What caused the actual TEMS hits, you night ask?

At an undetermined time, a lever arm on the second stage variable stator vane (VSV) broke and allowed the associated vane to operate independently of the other VSVs. The stalled engine actually experienced a total of seven actual TEMS 36 vibration hits on 13 prior missions, due to the third stage rotor blade vibrations. Before or during the mishap sortie, cracks developed at the base of the compressor blades as a result of high cycle fatigue. This all comes down to three things:

* How you take care of your tech data so you have the right information.

* How you interpret the information the systems provide you.

* How you, the worker, treat that information.

If you get information that says there is a problem, make sure you take all the required steps to ensure you have determined there is no problem and/or fixed the actual problem. It is easier to fix the small things than repair an entire engine.

Comedy Of Errors

An A-10 was launched as a weather ship to assess airspace conditions. If weather was sufficient, his wingman would launch, rejoin, and they would proceed on a local training sortie. However, the weather precluded tactical training and the aircraft proceeded single ship on an Advanced Handling Characteristics sortie. The mishap pilot performed several break-turn exercises, followed by two uneventful single-engine go procedures. On the third single-engine go exercise, the pilot received indications of high No. 1 (left) engine Inlet Turbine Temperature (ITT), generator failure, and hydraulic system failure. These indications are indicative of an engine failure. The pilot shut down the engine, requested assistance on the squadron common VHF frequency and coordinated a rejoin with another aircraft which was performing practice instrument approaches. The chase aircraft performed a battle damage check and assisted with checklist procedures and Fighter Resource Management. The pilot flew a single-engine approach and landed uneventfully.

Maintenance review of the Turbine Engine Monitoring System (TEMS) data pointed to a compressor stall as the initial cause of the mishap sequence. However, 52 days after the mishap, further review by the depot disregarded the compressor stall theory and pointed towards a flameout followed by a subsequent overtemp condition which caused the Class B damage. The unit shipped the mishap engine (ME) to the depot repair facility. Upon exterior inspection, three separate issues were found on the ME.

1. The B-nut securing the PS3 sensor to the top of the ME was found backed-off.

2. The canon plug connection between the T5 amp and the main fuel control (MFC) was discovered to be cross-threaded and apparently tightened with tools (against common maintenance practice of hand-tightening).

3. The MFC serial number did not match the serial number documented as being installed.

These three items, although separate in nature, proved to be directly related to the cause of this mishap. Interior engine inspection revealed significant damage to the engine aft of the combustion chamber. The HPT and LPT were completely destroyed along with the engine bearings and associated hardware parts as a result of severe overtemp conditions. Portions of the HPT had melted into molten metal and passed to the LPT and into the tailpipe causing extensive interior object damage (IOD). In addition, the outer seals on the LPT jammed as a result of the IOD and subsequently seized the engine.

TEMS data revealed the aircraft never flew in the engine disturbance envelope and the throttle was properly rigged. Investigation revealed no FOD or hardware failures occurred in the engine prior to the mishap sequence. However, lack of fuel was a factor. The fuel supply tested good post-mishap. The MFC was suspected as a cause during the compressor stall portion of the mishap investigation, and was PQDR’d. Analysis showed it was functioning as designed during the mishap sequence. In addition, the canon plug connects important signals from the T5 Amp to the MFC. However, post-mishap analysis proved the canon plug functioned despite its substandard connection. However, the loose PS3 B-nut found on the engine during disassembly contributes signals to the MFC, affecting fuel scheduling. Investigation revealed that a loose PS3 would have provided erroneous signals to the MFC, which would have caused a reduction in fuel flow to the engine below a level required for engine operation.

The investigation turned next to the cause of the overtemp sequence. The TF-34 maintains logic which, when the throttle is set at idle and the engine reduces core RPM below 56 percent, will automatically cause an auto-start attempt. This auto-start procedure would ignore the erroneous signals which caused the flameout and subsequently flood the combustion chamber with fuel to re-light the engine. This excessive flow of fuel caused the overtemp condition, which then caused the Class B category damage.

The key to the mishap remains highlighted by the undocumented MFC change. An MFC change would affect both the PS3 sensor B-nut and the T5 Amp canon plug connection. As mentioned earlier, both of these items were found and criticized for substandard maintenance practices (especially the canon plug). An MFC change would have forced the loosening (and hopefully, the re-tightening) of the PS3 B-nut. Also, the connection between the T5 amp and the MFC would have been loosened and re-tightened. However, it is impossible to investigate this change, since it was undocumented and could have occurred at anytime between the overhaul in 2000 and the mishap flight. There are no indications as to who, when, or why the MFC were switched between the ME and a second engine. Does this sound like quality maintenance? Not to me. We must document everything we do and follow the rules established to prevent mishaps.

COPYRIGHT 2004 U.S. Air Force, Safety Agency

COPYRIGHT 2005 Gale Group