Flying on by Andrew

The reduction in the number of centre barrel replacements (CBRs) comes in the wake of extensive structural testing by the Defence Science and Technology Organisation (DSTO), Director General Technical Airworthiness (DGTA) and the Defence Materiel Organisation (DMO) on 10 former US Navy, Canadian Forces and RAAF centre barrels, which found the aircraft’s key structural component had a longer fatigue life than first thought.

The challenging Hornet Upgrade (HUG) program is arguably the most comprehensive upgrade of any combat aircraft ever undertaken, with all of the aircraft’s communications, sensors, avionics, and electronic warfare systems updated, new and advanced weapons added, and substantial structural repairs and modifications performed. The reasons for performing the upgrades were two-fold: to maintain a qualitative edge over regional capabilities and possible near future adversaries; and to ensure the Hornet fleet safely reach their planned withdrawal date.

“It was in 2006 that we increased the scope of the centre barrel program to up to 49 aircraft, because the planned withdrawal date of the aircraft kept being extended,” the DMO’s director general aerospace combat systems, AIRCDRE Roy McPhail, told Australian Aviation. “In 2001 the planned withdrawal date was 2012, and in those days, I was hoping that just a few engineering changes would have meant we could have avoided doing any centre barrels. And then the JSF decision came out, and the planned withdrawal date was changed from 2012 to 2015. ” The requirement to replace the centre barrels had been based on a joint Australian and Canadian structural test program.

“The fatigue test showed the centre barrel had a certain life – you don’t actually see it starting to crack, but you know from the fatigue test that, at a certain fatigue life we can no longer guarantee the structure’s integrity,” AIRCDRE McPhail explained.

“There are five variables in fatigue life; your rate of effort, how hard you fly the aircraft, the planned withdrawal date, the level of risk, and how many repairs you do. ”Corrosion was also being found on the Hornets in areas away from the centre barrel as fatigue related repairs were carried out during major services. Indeed, up until 2001, the RAAF had never done deep maintenance on the F/A-18, as it was designed to never require such work. But it also wasn’t designed to be operated as hard and as long as the RAAF had been and intended to.

“We would rarely disassemble the aircraft and look inside. When conducting upgrades such as the electronics upgrade we would open up the aircraft and find some corrosion,” AIRCDRE McPhail said. “We were never supposed to do deep maintenance on the aircraft, so we never really got involved with it until 2001. ” But the decision to set the number of CBs to be replaced only came about after the DSTO were able to perform tests on CBs that had been replaced on US Navy, Canadian Forces and one RAAF aircraft.

“In the previous CBR strategy, if you conducted one test of one centre barrel and it fails at point X, you don’t know whether that particular centre barrel was a good one or a bad one,” explained AIRCDRE McPhail. “So we had to be conservative and assume that we fatigue tested a good one, so we set up a distribution curve to predict where the majority might fail. But because some may fail earlier, we needed to put the fatigue life early on the curve.

“However, a little over 12 months ago, DSTO took delivery of 10 old centre barrels that had been pulled out of US and Canadian aircraft as well as from the first RAAF aircraft to have its centre barrel replaced. They started testing the old centre barrels to see what they could do to give us an increased life. We told them we probably only needed another 10 per cent, which would effectively double the remaining life left. DSTO then came back to us earlier this year and told us it could be done, that they could give us our 10 per cent without increasing risk. Their findings underwent review by the DMO, RAAF and DGTA, and got the thumbs up. ” The DSTO testing involved pushing the centre barrels beyond their limits, using a rig with hydraulic rams.

“You can actually see the centre barrel move and flex, and you can hear it crack and groan,” AIRCDRE McPhail explained. “When you put an aircraft in for a full scale fatigue test, it’s an expensive activity, and when you have an event such as a crack occur, you don’t continue testing – you stop, and you repair that particular crack point, and then you continue. That’s what sets your life, because you know at that particular point that particular bit is going to crack. But what they did at DSTO was go way beyond that to see how far the crack would grow before the whole structure failed, and that’s where they made their gains. ” “Some areas of a centre barrel are uninspectable and unrepairable, and the previous argument was that it would be better to replace the whole structure than to try to work on it,” he continued. “But we’ve effectively been able to do a lot more fatigue work using surplus centre barrels which were already considered worn out under the old standards, and we found that we don’t have to replace them as soon as we thought. The 10 centre barrels have given us a much higher production representation, so we’ve found that our target ‘one in one thousand’ chance of losing an aircraft has moved further along the curve, thus giving us extra life.

“We did our analysis on the new figures, and we did a worst case analysis. We have 71 tail numbers out to 2018, and we run a fleet plan model where every tail number is tracked, when it’s going in for servicing and upgrades etc. Even in the worst case scenario where we get more airframe usage and burn more fatigue life, we can meet the requirements of the fleet through to planned withdrawal date. The Hornet is one seriously tough aircraft and the tests conducted by DSTO have proven that. ” AIRCDRE McPhail says the 10 CBRs currently contracted for would be sufficient to accommodate up to a two year slip in the planned withdrawal date. “We start to pull pilots out of the Hornet in 2014 to go to JSF, so in the end I’m going to have a lot of airframes with a little bit of life left on them, so there won’t be a shortage of airframes. ”

Hug rolls on

In other HUG program developments, the RAAF and its industry partners will continue to perform Structural Refurbishment Program (SRP1) work under Phase 3. 1 of Air 5376, right through until the 2013 timeframe. This involves the removal and repair of corrosion, the blending and machining of fatigue cracks, and the machining and sleeving of bolt holes and other key attachment points.

Further refurbishment work may soon be required on the Hornet’s wings as well.

“We’ve got some wing issues that are starting to emerge, so we’re trying to work out the size and scope of a wing refurbishment program,” AIRCDRE McPhail said.

“The US Navy is finding some stress corrosion cracking in some of their spars – we’re not finding them to the same degree in RAAF aircraft, and the cracks are not critical yet. It’s a matter of finding which wings need the work, and we may be asking to divert some of the savings from the centre barrel program to fund that work. We’ve also got some undercarriage issues we’d like to fund out of the savings as well. ” The Phase 2. 3 electronic warfare (EW) system upgrade is due to enter production next May after a successful series of prototype tests performed in Australia and at the US Navy’s test range at China Lake in California. “We’ll have three deliveries in capability,” AIRCDRE McPhail explained, with the full HUG 2. 3 EW capability to be fully fielded by September 2011. “We have what we call interim EW, where we have aircraft being modified now with just the (ALR-67(V)3) radar warner, but it’s not a mature type mod yet because we’re laying new wiring over old wiring on the first squadron of aircraft. We should have a squadron done by Christmas this year, and next May we’ll start with the mature hardware modifications where we put proper wiring looms in. They’ll actually come out of that mod with the radar warner and the new (wing pylon-mounted) BOL countermeasures dispenser system (CMDS), as well as the provisions for the external jammer pod. The BOL will give us a huge chaff and flare capacity, and that will be driven by the existing built-in ALE-47 CMDS. ” The new Elta jammer pod will be externally carried on either the centreline or outboard wing stations. “The jammer is a fantastic piece of kit and certainly gets a lot of respect from the operators. ” “The last stage of 2. 3 is when we get the final software – that’s when all the magic happens and we get it all to work together – this will be the most challenging part of the modification process. It will be a world class EW suite, and a big achievement for the project office when completed. ”