Potential issue for operators of CF6-50 powered freighters
We included this in last night’s issue of Cargo Facts Update (and yes, if you don’t already subscribe, you should) but I think it deserves wider notice, and I’d like to hear your comments on it, so I’ll put it up here.
In a move that has potential to impact the worldwide freighter fleet, the US National Transportation Safety Board (NTSB) issued an urgent safety recommendation to the FAA, asking the FAA to implement severe restrictions on the operation of aircraft powered by CF6-50 and CF6-45 engines. This includes A300B4s, DC-10-30s, and 747-100s/-200s/-300s – aircraft that are now primarily operated in freighter configuration.
The background to the recommendation is that in the last two years there have been four uncontained
CF6-50 engine failures (caused by failure of the low pressure turbine stage 3 disk), and although there have no injuries or crashes, the NTSB has determined that current regulations are not sufficient to prevent further failures. It has therefore recommended that, among other things, the FAA do the following:
- Immediately require operators of CF6-45/-50-powered airplanes to perform high pressure turbine rotor blade borescope inspections every 15 flight cycles.
- Immediately require General Electric Company to redesign the CF6-45/-50 low pressure turbine stage 3 diskso that it will not fail when exposed to high pressure rotor unbalance forces.
- Require all operators of CF6-45/-50-powered airplanes to install the newly designed LPT S3 at the next maintenance opportunity once it is available from GE.
The FAA is not required to act on these recommendations, but uncontained engine failures are not something easily ignored. If the FAA does implement the NTSB recommendations in the form of an Airworthiness Directive,
the impact on operators of CF6-50-powered freighters would be immediate and financially onerous.
The current active fleet of affected aircraft is roughly as follows:
A300B4Fs 45 (some of which may be Pratt-powered, and therefore not affected)
DC/MD-10-30Fs 35 (17 of which are operated by FedEx)
747-100/-200/-300Fs 75 (not all of which are CF6-powered)
Full text of the NTSB recommendation can be found at www.ntsb.gov/Recs/letters/2010/A-10-098-101.pdf
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One of the carriers that had an un-contained engine failure is based in Singapore. I looked at the engine after the event: yes it is true the entire LPT 3 disc separates from the hub and based upon the video boroscope evidence this particular situation was predicated by some very minor FOD ingestion that caused minimal impact damage in the Booster / HPC. The difficulty in this particular case was isolating the timeline between the initial FOD damage and the failure of the engine, less than 150 cycles had passed since the last boroscope and there were no abnormal engine trends in advance of the failure event (note that the 747-200F relies on manual engine health monitoring data recording). The engine failure mode is still being investigated by GE.
In this case the engine failure occurred on take off from Singapore and caused significant airframe and lower wing damage, the crew handled the failure in accordance with procedures and returned the aircraft to Singapore without further problem. This was yet another “safety of flight” event that demonstrated what a great aircraft the 747 is, all that redundancy designed and built into the aircraft by Joe Sutter and his team has paid off time and time again. Unfortunately the aircraft was deemed BER by the insurer because of the low aircraft value.
Whilst I tend to agree that the initial GE recommended repeat inspection frequency of 200 cycles was probably too high, 15 cycles on its own may be too low and will cause significant operational and engine maintenance cost issues. In conclusion I have the following observations:
1. It does seem odd that this failure mode has suddenly arisen, its significant in nature and apparently has no current fix; GE need to design a new LPT Disk as a matter of urgency and also determine if there are any other interim solutions to reduce both the in-service inspection burden and the increased engine shop visit maintenance cost that will result from these inspections.
2. A significant number of engines will require premature shop visits as the accumulated HPT 2 damage limits are now more restrictive due to the rotor imbalance issues. Operators will no doubt try to source spare engines first as this will be cheaper than shop visits, as there is no permanent fix available presently any operator completing a shop visit now knows he will have to do it again in the future to relieve the repeat inspection burden. CF6-50 engine values and demand will no doubt have a temporary spike and some operators will ground aircraft prematurely;
3. I think operators should address the NTSB recommendation when the FAA issue an NPRM amendment to the existing AD. It is probably sensible to look at accepting the 15 cycles as an initial threshold to be repeated say 3 times, after which the threshold should increase to say 50 cycles. At the same time the maintenance actions for any shift in engine parameters, particularly EGT and vibration should include an immediate T2 boroscope.
4. Operators should demand a meeting with GE to pressure them into: i) developing a new LPT 3 disk urgently; ii) continuing research into the failure mode and helping reduce the inspection burden; and iii) material credits for premature shop visit costs and FOC LPT 3 disc supply. Good luck with that one!