Flight International - 04

Programme Recovery
US Marine Corps
Flaw was identified in wake of a
crash involving STOVL variant
Engine fix ends F-35 delivery pause
Ten-week suspension of stealth fighter shipments is brought
to an end after Pratt & Whitney resolves vibration issue
Ryan Finnerty Tampa
The US Department of Defense
(DoD) has cleared
the resumption of Lockheed
Martin F-35 deliveries, which
had been on pause after a non- fatal
accident involving the type.
“[We] are currently working with
the US services, partner nations
and Foreign Military Sales customers
on the movement of aircraft to
their operational units,” the Pentagon’s
F-35 Joint Program Office
(JPO) said on 14 March.
The operational use of all F-35s
received approval from the DoD
after “extensive technical and
flightworthy checks”, it adds.
Acceptance of new-build aircraft
had been placed on hold after a
short take-off and vertical landing
(STOVL) F-35B crashed during a
test flight at Lockheed’s Fort Worth
site in Texas last December.
The DoD and Lockheed subsequently
grounded all F-35s with
less than 40h of operational use
and halted new deliveries.
F135 engine supplier Pratt & Whitney
(P&W) traced the mishap to a
“harmonic resonance”, or vibration
issue. Jennifer Latka, its vice-president
of F135 programmes, says the
issue occurs at “a certain frequency
and certain amplitude” when multiple
parameters converge to create
a “very rare systems phenomenon”.
Having identified the cause in
February, P&W developed a “mitigation
measure”, enabling it to
resume F135 deliveries, and production
flight operations at Fort
Worth to begin again on 6 March.
The DoD has ordered a fleetwide
modification, which the JPO
says involves an “inexpensive, nonintrusive”
retrofit which can be performed
at operational units in 4-8h.
Lockheed notes that it continued
producing jets during the 10-week
flight pause, and says the disruption
will not prevent it from meeting
contracted delivery obligations.
The airframer last year missed its
F-35 delivery target by seven jets,
which chief financial officer Jay
Malave attributes to the temporary
delivery suspension. Having handed
over 141 of the aircraft in 2022, it this
year plans to transfer 156 examples.
Propulsion upgrade
Meanwhile, the US Air Force
(USAF) has supported upgrading
the current propulsion system on
its F-35As, rather than purchasing
a replacement engine.
Announced by secretary of the air
force Frank Kendall on 13 March, the
decision came as the Biden administration
released its proposal for
military spending in fiscal year 2024
without funds for the Adaptive
Engine Transition Program (AETP).
“[AETP] is not transitioning to a
programme of record,” the USAF
confirms. Under the effort, the service
in 2016 began funding development
of a new engine for the
F-35, centred on an adaptive-cycle
design capable of boosting electrical
power generation and thrust.
The service spent some $4 billion
on the effort, with both P&W and GE
Aerospace producing prototypes.
GE had urged the USAF to adopt
its XA100 design, while P&W –
despite developing an XA101 prototype
– instead promoted a third
option: an engine core upgrade
(ECU) for the F135.
A major factor behind the air
force’s decision appears to have
been the AETP engines’ lack of
suitability for programme-wide integration,
specifically on the F-35B.
“Although the results of analyses
determined AETP provided
the best overall F-35A operational
performance, the F135 engine core
upgrade will restore engine life and
prevent degradation for all three
F-35 variants at the lowest cost,”
the service tells FlightGlobal.
“We can deliver upgraded engines
starting in 2028,” says Jill Albertelli,
president of P&W’s military engines
division. “The F135 ECU saves billions,
which ensures a record quantity
of F-35s can be procured.”
GE has criticised the decision.
“This [FY2024] budget fails to
consider rising geopolitical tensions
and the need for revolutionary
capabilities that only the XA100
engine can provide by 2028,” it says.
The USAF’s preference is no
guarantee, however, with lawmakers
to have the final say.
“Nearly 50 bipartisan members
of Congress wrote in support of
advanced engine programmes like
ours because they recognise these
needs,” GE says. ◗
8 Flight International April 2023