Top 10 list of F-35B flaws and fixes

Do not accuse the F-35 team for a lack of transparency. On the contrary, in recent interviews, Lockheed Martin and Pratt & Whitney have identified at least eight things wrong with the short take-off and vertical landing (STOVL) F-35B, which I have covered here and, more recently, here.

In the past several days, events have supplied two more open questions — affordability and power generation. A couple of these issues, such as wing roll-off and generators, apply also to the conventional take-off and landing (CTOL) F-35A.

As I explained in my most recent article, these represent both the threats and the keys to the F-35B’s survival. The flaws have created the threat in the form of the F-35B’s two-year probation status by the US Department of Defense, which was timed only a few weeks after the Royal Navy dropped out of the program. But the key for the F-35B’s survival will depend on how well Lockheed’s proposed fixes solve the problems.

It’s still very early in the flight test phase, of course, so Rumsfeld’s infamous “unknown-unknowns” are also still out there. A possible example are concerns about the F-35B’s power and thermal management systems. In his farewell interview with Flight International, former chief test pilot Jon Beesley identified thermal management as a key concern heading into tests this summer with mission systems aircraft, which includes the STOVL BF-4. But that isn’t officially a problem yet, so it’s not on this list.

In no particular order, this blog’s current list for the top 10 list of F-35B problems and solutions are on the jump:

1. Bulkhead cracks

The Problem: It was revealed in November that the aluminum alloy 496 bulkhead cracked less than 10% through a 16,000hr durability test.

The Solution: In the short-term Lockheed has redesigned a hard edge for the bulkhead into a curve. By redistributing the load, the 496 bulkhead will remain airworthy for 1,500h. That’s plenty of time for the test fleet, but operational aircraft need a load-bearing bulkhead to survive at least 8,000h. As a result, Lockheed is planning to install a 7-8lb steel patch on the bulkhead for production aircraft. A composite patch is also being studied.

2. Vertical lift bring-back (VLBB)

The Problem: As Lockheed defines it, the F-35B right now has all the vertical thrust it needs to “bring back” the required load of weapons and fuel onto an amphibious carrier. But today’s lifting capacity won’t be enough as the F-35B’s weight is expected grow like any other operational aircraft.

The Solution: It’s not exactly clear how much weight is in the trade space. Lockheed only says that 400lb is “a lot” in the context of a VLBB debate. Meanwhile, Pratt & Whitney has offered to increase the vertical thrust of the F135-PW-600, which is the STOVL variant, by around 200lb, if necessary. Lockheed prefers to reclaim the VLBB growth margin by putting the F-35B’s structures and systems on a diet.  

3. Auxiliary air inlet (AAI) doors

The Problem: Three doors open to allow air to flow into the F-35B’s engine. The big one is the one-piece door after of the cockpit — internally nicknamed the ’57 Chevy hood. Just behind that doors are the auxiliary air inlet doors. As the amount of ram air declines as the F-35B slows to a hover, the auxiliary inlet sucks more oxygen into the liftfan engine, which increases vertical overall thrust. The problem is that the hinges for the doors are too weak for the turbulent air at 250kt.

The Solution: The near-term fix is to change the software so the AAI doors can’t open until the F-35 slows down from 250kt. The long-term fix is redesign either the doors or the hinges to survive the turbulent air at higher speeds. Lockheed believes this will be a relatively simple fix because both the doors and hinges are easily accessible.

4. Parts reliability

The Problem: The F-35B’s parts reliability is poor, even for a program at this stage of development. Complaints have been hears about everything from key components of the propulsion system to the rudder pedals.

The Solution: In the short term, Lockheed is simply ordering more parts stockpile. As this tends to increase costs without addressing the root problem, Lockheed also has a long-term plan. In the latest restructuring unveiled in January, Lockheed and it’s key suppliers will receive new contracts to make investments to improve reliability of the thousands of parts and components.

5. Wing roll-off

The Problem: At transonic speeds — generally between 0.95M and 1.05M — air flow for any supersonic fighter starts getting “squirrelly”, as pilots call it. In this regime, air can be flowing over one wing or parts of one wing at supersonic speed, while moving subsonically on the other wing. This is especially true when the fighter is maneuvering aggressively. In such disruptions, one wing has a tendency to “roll-off”, a movement not quite as severe as the “wing drop” problem experienced by the Boeing F/A-18E/F Super Hornet and still on the list of concerns for the F-35C carrier variant. The F-35B has experienced wing roll-off in flight tests, which Lockheed says was expected.

The Solution: The F-35 is a fly-by-wire aircraft, so the plan is to counter the wing roll-off with a software change that sharpens the responses by the flight controls when such events occur. This is not expected to solve the problem completely, but it needs to be better. “You’re never going to be perfect in that regime, but itneeds to be acceptable,” Lockheed says.

6. Driveshaft:

The Problem: The driveshaft that transfers power from the engine to the liftfan met Lockheed’s performance target, but flight tests revealed the original requirement wasn’t enough, according to Pratt & Whitney. As a result, the driveshaft is stretching and compressing at higher than expected levels.

The Solution: The near-term fix has been to insert spacers at each end of the shaft, allowing the component to expand more as heat rises. In the long-term, P&W is redesigning the driveshaft to meet higher tolerances for expansion and contraction.

7. Roll-post nozzle:

The Problem: Actuators the move the roll-post nozzles embedded inside each wing are burning out faster than anticipated. The cause is not the design of the actuator itself. Instead, the temperatures inside the wing are hotter than expected, so the actuators are exposed to unexpected levels of heat.

The Solution:  In the near-term, more insulation is being installed around the actuators, and ventilation in the wing is being improved. In the longer-term, Pratt & Whitney will upgrade the actuator design to survive more hotter temperatures.

8. Lift-fan clutch

The Problem: According to Pratt & Whitney, it happens “very infrequently”, but somehow the clutch plates for the lift-fan touch in flight. This creates friction that exceeds the clutch’s temperature limit.

The Solution: Pratt & Whitney is still trying to understand the source of the problem, so the company is installing a sensor in the clutch. This should explain what is causing the problem. It will also tell the pilots when clutch plates are over-heating, so they can fly to a cooler portion of the flight envelope.

9. Generators?

The Problem: The root cause is still being investigated, but a CTOL F-35A flight test aircraft lost both main power generators on 9 March. The integrated power pack kicked in, which gave the pilot enough power to steer the AF-3 mission systems aircraft back to landing at Edwards AFB. 

The Solution: It’s too early to tell if the problem is isolated to the CTOL variant, or even a manufacturing issue with the AF-3 in particular. We should know more fairly soon, perhaps even later today.


10. Price tag?

The Problem: The Royal Navy has already bailed on the F-35B. The US Marine Corps has transferred orders for 60 F-35Bs into F-35Cs. The near-term production schedule has been slowed to a trickle. And development years longer and billions more costly than anticipated.

The Solution: This is going to be tricky. Neither the Royal Navy nor the Marine Corps has canceled overall aircraft orders, and the F-35C’s price tag is likely to decline. How that impacts the final cost of the F-35B has yet to be determined. In the meantime, both Department of Defense and Lockheed officials say they are working to reduce the cost of all three variants by investing in a set of cost reduction initiatives.


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2 Responses to Top 10 list of F-35B flaws and fixes

  1. Robert Seth Tomlinson 21 March, 2011 at 1:15 am #

    Why can’t the AV8 Harrier be modified with F-35 technology?

  2. Bjørnar Bolsøy 22 March, 2011 at 8:09 pm #

    Nice recap and graphics. Would be interesting to know what the other items are.

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