PARIS --- The F-35 Joint Program Office’s Feb. 28 decision to allow the F-35 fleet to resume flying is explained, according to media reports, by the fact that no other turbine blade cracks have been discovered, which is taken to imply that no engine redesign is needed.
Engine maker Pratt & Whitney earlier had ruled out a high-cycle fatigue crack, which would have pointed to more serious, systemic problems.
Matthew Bates, a spokesman for Pratt & Whitney, was quoted in a March 1 Reuters story as saying that the “crack resulted from the ‘unique operating environment’ in flight tests," many of which tested the engine's powerful after burners, Reuters said.
“Bates said the engine in question had operated at high temperatures for more than four times longer than a typical F-35 flight, which led to a separation of the ‘grain boundary’ of this particular blade,” Reuters added.
The JPO’s statement, again according to Reuters, said the investigation concluded that the 0.6-inch long crack was caused by "prolonged exposure to high levels of heat and other operational stressors."
(Neither the JPO nor Bates had responded to our requests for information by our deadline, made well after hours.)
In other words, the F-35 cannot safely operate in afterburner mode for somewhat extended periods – we do not know how long – before turbine blades start breaking apart which, given that the F-35 is single-engined, is likely to prove unwelcome to pilots.
But this is not the first instance in which the F-35 has run into problems when using its afterburner.
In his annual report released Jan. 15 Michael Gilmore, the Pentagon’s undersecretary for Operational Test and Evaluation, mentioned two instances of afterburner-induced incidents discovered in 2012:
1. F-35A Flight Sciences Assessment (Page 29):
“The test team could not execute this portion (30 percent) of planned 2012 baseline test points [because of] aircraft operating limitations, which prevented the extended use of afterburner needed to complete high‑altitude/high‑airspeed test points,” and
2. F-35C Flight Sciences (Page 33):
“In August, the test team installed new coatings on CF-1 horizontal tails, designed to prevent scorching and delaminating during prolonged use of afterburner pursuing high airspeed test points. However, portions of the coatings dis-bonded during flight, suspending further testing of the high airspeed portion of the envelope.”
The F-35 section of Gilmore’s report can be accessed here on the DOT&E website.
We already knew, thanks to Gilmore’s report, that the F-35 cannot currently fly “within 25 miles of known lightning conditions” and that “below 20,000 feet altitude, descent rate is restricted to 6,000 feet/minute (….) Neither restriction is acceptable for combat or combat training.” (see page 41)
Now, thanks to this latest grounding, we discover that the F-35 is also limited in the time it can operate on afterburner, at the risk of having its turbine blades explode. This can hardly be considered acceptable for combat, either.
Neither Pratt & Whitney nor the JPO appear overly troubled by this limitation, and neither considers it necessary to redesign the engine or the turbine blades to avoid a recurrence. The only known precautionary measure, again reported by Reuters, is that the JPO will “now require reports to monitor and limit similar damage after every 25 flight hours,” instead of 50-hour inspections as previously.
So, 11 years into its development program, the aircraft can’t fly near thunderstorms, it can’t dive too steeply, and it can’t sustain afterburner operation.
As the turbine blade cracked after 409 flight hours (and 700 total operating hours), one possible fix is to change engines every 400 hours. After all, this was standard operating procedure in the Soviet air force, as Germany’ Luftwaffe found when it inherited the MiG-29 operated by the previous East Germany.
However, given the F-35’s already high operating costs, this is probably not a route that Lockheed or the Pentagon would advocate. Yet, in some respects, the F135’s operational lifespan in afterburner mode takes us back to the 1980s.
This is hardly a badge of honor for the much-vaunted fifth-generation fighter.
Update: The third paragraph was amended March 1 to correct a quote by P&W spokesman Matthew Bates at his request -- Editor.