Cracking up: US Air Force C-17s, F-15s, F-16s etc …

More bad news for the US Air Force’s aging aircraft fleets.

I’m sure everybody remembers the infamous longeron-cracking that grounded the entire Boeing F-15 fleet in November, which for a time left (gasp!) Oregon’s air space briefly undefended.

Less strenuously reported is the worsening structural condition of growing numbers of older-model Lockheed Martin F-16s. Cheers to Graham Warwick for forwarding this snippet of written testimony from a Congressional hearing earlier this month. The writing is attributed to “Daniel J. Darnell Deputy Chief of Staff Air, Space and Information Operations, Plans and Requirements Headquarters, U.S. Air Force.”

He says:

Wing pylon rib corrosion, a known problem with the F-16 aircraft is an issue we monitor closely. This corrosion prevents the F-16s from carrying pylon mounted external fuel tanks, which limits their effective combat range. While we currently have three F-16 aircraft grounded due to wing pylon rib corrosion, the corrosion problem is somewhat common across the fleet. For example, within the past 24 months, we identified 27 aircraft at Aviano Air Base, Italy with this problem. We currently inspect F-16 aircraft every 800 hours to monitor for this problem. Because of inspections, we have also found approximately 16% (69 of 399) of our Block 40/42 F-16 aircraft now have bulkhead cracks. This discovery has led to 22 Block 40/42 F-16 aircraft grounded due to the severity of the cracks. An additional 41 aircraft continue to fly with flight restrictions. We will continue to monitor this situation closely.

Now comes the news this week that the relatively young C-17 fleet is also dealing with a structural problem.

General Arthur Licthe, chief of Air Mobility Command, told lawmakers this week that stress imposed by the C-17′s thrust reversers have spawned fuselage cracks in the airframe. I asked Boeing for elaboration, and here’s what they told me:

Thanks for your question on the C-17 fatigue cracks. The stress cracks, beneath and forward of the wings on the C-17 fuselage are a minor occurrence and don’t pose a safety hazard for the aircraft or flight crews. Boeing has developed a fix for the problem and the fix has been demonstrated and validated to meet the durability requirements. There’s no impact on the operational readiness of these planes and the cracks are being fixed during regularly scheduled maintenance with a simple technique.

I also asked Boeing why the thrust reversers would lead to fuselage cracks forward of and beneath the wings. Boeing’s reply:

It’s referred to as the “fire hose effect.” The reversed air, from the fan thrust reverser, glances off the fuselage forward and below the wing. The resultant stresses in the fuselage skins due to this impingement were higher than anticipated and have resulted in the small skin cracks.

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4 Responses to Cracking up: US Air Force C-17s, F-15s, F-16s etc …

  1. ELP 4 April, 2008 at 4:10 am #

    Yikes! Thanks for the info guys.

    Yeah C-17s were made to reach a weight target… and well, that required some light material setups.

    They do require some work when they show up at the depot for a refirb, but given the ops they do all the time…

    Funny as when the C-17 was first being pitched to the USAF, it was stated that it would never be a PDM (Periodic Depot Maintenance) jet. Sounds nice until you start using them and well, now they go to the depot just like a C-5 or C-130.

    I expect the C-17s will age just like the C-141s. Look at the life of the C-141. It was used and used and used and the last few years they were pretty thrashed. Thats life.

    Scary about the Block 40/42 F-16s. A Block 25 just passed 7000 hours recently.

    I think there will be more discoveries with F-16s as well, it’s only natural. Hey, like the F-18s the designers never had a clue they would be used this much. Just use them up to the airframe life and toss them in the trash. Now years later, here we are pushing F-16s that were designed for 4000 hours out to 8000 if possible and for the Navy, doing CBR (center barrel replacement) on the F-18s when they start looking bad at 4500-5000hrs or whatever. Even then for F-18 classics, a CBR doesn’t address all the wear: airframe cracks in other places, that are a case by case basis. They say the Super Hornet is good for 6000hrs. If JSF goes wobbly we might push Supers out that far for sure even after buying more to fill the breach.

    I wonder if one would have got some funny looks from the engineers back in the day when the light weight fighter program was thought up by stating you wanted to keep refirbing the airframes after they reached their original design limit.

  2. RTLM 4 April, 2008 at 4:51 am #

    Well – at least “skin cracks” sound better than “cracks in the airframe”. Maybe put some blast resistant material on that section of the fuselage…

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