KESTEREN, Netherlands --- Last week’s grounding of the F-35 fleet puts the Pratt & Whitney F-135 engine back in the public spotlights, just one week before the planned international debut of the F-35B at the RIAT 2014 and Farnborough Airshows.
During 2007-2009, repeated problems with turbine blades contributed to significant delays in the F-35 test program, and required a partial redesign of certain parts of the engine. The grounding in February 2013 was caused by a crack, found in a Low Pressure LPT3 turbine blade. Investigations of the latest engine fire seem to focus at the same problem.
The question is whether these recurring F135 problems should be considered as isolated incidents, or as signs of a basic design fault? And do F-35 operators have a structural problem?
Long history of engine problems since 2006
It cannot be excluded that the root cause of the current problem is more structural than a simple manufacturing error or an isolated incident. Since 2006 there has been a series of engine problems with the F-135 engine.
In May 2006, Aviation Week reporter David A. Fulghum wrote a detailed article “Joint Strike Fighter F135 Engine Burns Hotter Than Desired” and described the risk of a shorter engine life or engine damage caused by higher than expected temperatures on the F-135 engine.
In August 2007 and February 2008 there were serious problems. Turbine blades broke off suddenly by a form of metal fatigue. The cause was sought in a combination of factors. On 30 August 2007 in test engine FX634, after 122 hours of testing, a turbine blade in the 3rd LPT stage broke off completely.
On February 4, 2008 something similar happened to engine FTE06, also in the 3rd LPT stage, after 19 hours.
At the time, the JSF Program Office told the press that the engine failures in both cases were due to “high cycle fatigue testing”. These problems with the engine contributed significantly to the delays in the JSF test program for the period 2007-2008.
Redesign of the engine in 2008
In early 2008 an engine, the FX640 ground test engine, was equipped with numerous sensors and instruments. On April 21, 2008 a test process was started to find the cause of the problem. Through a detailed test plan the forces and tensions that arise in the engine were mapped in different power ranges.
At that moment it seemed to be primarily an issue of the F-35B STOVL (vertical landing) version. The cracks in the turbine blades were created in exactly the same place, and seemed to occur when switching from forward to vertical drive. Later in 2008, the results became available. The blade cracks seemed to have been caused by certain vibrations that triggered a material failure.
This led to a redesign of a number of elements in the engine. One of the upgrades was a change of the distance between the turbine blades. After the redesign, the engine was retested and recertified. At the end of 2008 Pratt & Whitney issued a press statement, saying that they were convinced that the problems were solved.
In 2009, problems with redesigned engine
During testing in May 2009, Pratt & Whitney found that at high speed with full after burner and at low altitude, certain pressure pulsations occurred. This “screech” problem, that prevented the engine from sustaining full thrust, was addressed by modifications in 2010 and included design modifications in the fuel system, upgraded software and reductions of aerodynamic leakages.
In July 2009, the then head of the JSF Program Office, Marine Corps Maj. Gen. David R. Heinz, was still was not happy with the F-135 problems. He told the press: “The problems include too many individual blades that fail to meet specifications, as well as combined “stack-ups” of blades that fail early. I’m not satisfied with the rates that I’m getting.”
A few days later he was ordered by the Pentagon not to comment publicly on problems with the F-135 engine.
On September 11, 2009, again serious engine problems were revealed during testing of the Pratt & Whitney F-135 engine. At a crucial moment in the debate in the U.S. Congress on the choice of two competing engine types (the Pentagon wanted to axe the GE / Rolls Royce F-136 alternate engine), a Pratt & Whitney F-135 engine broke down. Again, the cause seemed to lie in broken turbine blades. However, this time the same problem occurred in the new, redesigned engine with redesigned turbine blades.
Pratt & Whitney stated that a defected bushing led to damage of the some fan blades. Pratt & Whitney also announced that a “minor modification” would be incorporated in all ISR (Initial Service Release) engines.
Engine problems continuing in 2011
After the problems in 2009, program officials no longer publicly commented about the engine problems. Also, there were no indications of any recurring problems with the engine; or that there were any reliability issues.
In April 2011, however, Admiral Venlet, then Head of JSF Program Office, told reporters that some engine problems were impacting on the delivery schedule. Pratt & Whitney confirmed to the press that “a small number” of F135 engines had been replaced with spares since March 2011 “with no impact to the F-35 test programme.” These replacements were ordered after detection of a mis-assembled ground test engine and further checks had identified the same problem on other (production) engines.
Two groundings in 2013
The F-35B STOVL variant was grounded Jan 18, 2013 after detection of a failure of a fueldraulic line in the aircraft’s propulsion system. The Pentagon cleared all 25 F-35B aircraft to resume flight tests on February 12, 2013. Pratt & Whitney engineers diagnosed the problem as a crimp in one of the fluid lines of the fuedraulic system, which is a system that uses jet fuel (rather than standard hydraulic fluid) to lubricate mechanical parts.
A more serious issue was found when on February 19, 2013 a routine inspection took place of a Pratt & Whitney F135 engine at Edwards AFB, USA. During the inspection using a borescope, there were indications that there was a crack in a LPT turbine blade. It was confirmed after further investigation. The turbine blade was sent to Pratt & Whitney in Middletown (CT), USA for further investigation.
On Thursday, February 21, 2013, the Pentagon Friday ordered the grounding for all F-35 aircraft. The F-35 JSF Program Office said in a Feb. 22 statement to the press: “It is too early to know the fleet-wide impact of this finding; however, as a precautionary measure, all F-35 flight operations have been suspended until the investigation is complete and the cause of the blade crack is fully understood.”
Some facts about the February 2013 incident
Involved in the February 2013 incident was the tenth F-135 engine with 700 hours, of which 409 flight hours. The aircraft was the F-35A test aircraft AF-2. The half-inch wide crack was found in a turbine blade of the low pressure turbine section. This makes it unlikely that it is caused by so-called FOD (Foreign Object Damage), such as a bird strike, because such an object has to pass the Fan Section (3 stages) Compressor Section (6 stages), combustor and high pressure turbine section before reaching the low pressure turbine section.
March 6, 2013 the JSF Program Office told the press that the problem was caused by thermal creep from stressful high-temperature, high-intensity testing at supersonic speeds and at low altitudes for a prolonged period of time, generating significantly more heat than expected.
New significant test failure December 2013
On December 23, 2013 ground engine FX648 experienced a “significant test failure” during accelerated mission tests (AMT) at Pratt’s West Palm Beach facility.
The engine suffered a failure of its 1st stage fan integrally bladed rotor (IBR, also known as a “blisk”) while doing ground accelerated mission durability testing. The stages are made up of integrally bladed rotors (IBR), the first of which is constructed from hollow titanium (the second and third are made of solid titanium).
The engine involved was the highest-time F135 in the test fleet, with about 2,192 hours of running time, or approximately nine years of service as a test engine -- more than four times the hours of any operational F-35 engine (By comparison, the high time SDD flight test engine has 622 flight hours and the high time operational engine has less than 250 flight hours).
This event was revealed months later, on March 26, 2014, by F-35 Program Executive Officer Lt. Gen. Christopher Bogdan during testimony to the US House of Representatives. Later, he told the press that “they had underestimated the stress at low-cycle fatigue.”
Pratt & Whitney said in a statement: “Our investigation is ongoing, but we have determined this incident does not pose a flight safety risk and will have no near-term impact to the operational fleet.” In April 2014 the root cause of the problem was still unknown.
In-flight emergency F-35B after major oil leak - June 2014
A new fleet-wide grounding order was issued on June 13, 2014 after an in-flight emergency with an F-35B. The pilot landed safely at Marine Corps Air Station Yuma (Ariz.) after what NAVAIR said in a statement was “a major engine oil leak, the source of which appears to be a separated oil inlet line from the oil flow management valve (OFMV) Rosan fitting. The fitting is common to all F135 engines.”
After engine-by-engine checks, most of the F-35 fleet (104 units at that time) was cleared to fly again some days later, but two other F135 engines were declared to have “suspect findings”.
F-35A with extensive engine fire at Eglin – June 2014
On Monday June 23, 2014 at 9:15 p.m. an F-35A assigned to the 33rd Fighter Wing’s 58th Fighter Squadron at Eglin Air Force Base caught fire as the instructor pilot was taking off as part of a two-ship formation for a continuation training mission. First reports said the “significant fire” originated in the tail of the aircraft, mentioning it a Class-A (big) incident.
The pilot successfully shut down the plane and escaped unharmed and the fire was extinguished with foam by a ground crew. The aircraft involved is the AF-27, s/n 10-5015, a LRIP-4 series aircraft that made its first flight on April 22, 2013 and was delivered on May 29, 2013 to the US Air Force. The F-35A was towed to a hangar. Accident investigators have collected any related foreign object debris at the same hangar for review.
No any pictures are known or published of the damaged F-35A. Unconfirmed reports claim the F-35A will be written-off.
Later, one eye-witness said, according to some press reports, that “The engine ripped through the top of the plane.” And, about debris was found on the runway six feet around the aircraft.
All F-35A flight operations were temporarily suspended at Eglin as they investigated the nature of the incident, but flight operations elsewhere continued.
However, after a week the Pentagon said July 3 it still had not found the cause of the fire, that the engine was the cause (not the Integrated Power Pack) and the technical air worthiness authorities of the Department of the Air Force and Department of the Navy issued a directive to ground the F-35 fleet based on initial findings from this incident. Additional inspections of F-35 engines had been ordered, and return to flight would be determined based on inspection results and analysis of engineering data.
The investigation is said to be focused – again - to the third stage turbine of the F135 engine as the likely source of the fire. The third stage turbine is the second stage in the low-pressure turbine section and common to all F-135 variants – the F-35A, F-35B and F-35C versions.
Preparations continued for F-35 participation in two major air shows in the United Kingdom, RIAT 2014 at RAF Fairford and the Farnborough Air Show. A final decision is expected July 10, 2014.
A spokeswoman of the Joint Program Office (JPO) told IHS Jane’s that they had “temporarily suspended” negotiations for the next lot of F-135 engines and that negotiations about the LRIP-8 series of F135 engines would resume when the scope of the latest engine issue and downstream effects would be known.
(The LRIP 7 engine contract was due to be awarded in fall 2013, as the LRIP 7 airframe contract was awarded on Sept. 27, 2013. The latest engine contract awarded to P&W, on Oct 23, 2013 is for LRIP 6 engines—Ed).
The repeated problems with the same part of the engine may be an indication of a serious design and structural fault with the F-135 engine.
A future F-35 fleet-wide grounding will paralyze Western airpower. Also, the lack of reliability will contribute to low service ability and to higher operating and support costs of the F-35 fleets in several countries, putting more pressure on low defense budgets.
Since the F-35 will be the cornerstone of NATO airpower and US homeland defense for the next decades, the problems with the F135 engine need the attention of the highest political decision-makers.
BACKGROUND: History of previous F-35 groundings
May 2007 (electrical system, engine): The first incident was recorded in May 2007, when the F-35A prototype AA-1 experienced an electrical short that disabled flight controls on the horizontal stabliser. A grounding was ordered and continued until December 2007, due to time needed to redesign several parts of the 270-volt electrical system and F-135 engine problems.
July 2008 (cooling, electrical): On July 23, 2008, both flying F-35 prototypes were grounded after problems were detected with ground cooling fan electrical circuitry, DCMA reported on Aug 18, 2008 that tests were delayed as a result of testing anomalies on the 28 Volt and 270 Volt Battery Charger/Controller Unit, the Electrical Distribution Unit and the Power Distribution Unit. It was due to design problems. Flights were resumed first week of September-2008.
December 2008 (engine, ejection seat): On Dec 12, 2008 the F-35 was grounded again as a result of engine and ejection seat anomalies. Seat anomalies were observed in ejection seat sequence during an escape system test on Nov. 20, 2008. It took nearly 3 months to solve the problems and aircraft AA-1 did not return to the skies until Feb. 24, 2009.
May 2009 (most likely engine): The F-35 fleet didn’t fly between May 7, 2009 (84th flight of prototype AA-1) and Jun 23, 2009, shortly after reports of new engine problems (the “screech” problem). No comments were available from JPO or L-M.
October 2010 (engine, fuel pump): F-35 fleet grounded after the fuel pump shut down above 10,000ft (3,050m). A fuel pump sequence error, caused by a software bug, could have initiated an engine stall.
March 2011 (Integrated Power Package): The entire F-35 fleet was grounded some weeks after test aircraft AF-4 experienced a dual generator failure on March 9, 2011. After both generators shut down in flight, the IPP activated and allowed the F-35’s flight control system to continue functioning. The problem was traced to faulty maintenance handling.
June 2011 (software): Carrier-based F-35C suspended from flying after engineers at NAS Patuxent River discovered a software problem that could have affected the flight control surfaces. Grounding was from 17 June until 23 June, 2011.
August 2011 (Integrated Power Package): A precautionary grounding of all 20 F-35s that had reached flying status was ordered Aug. 3, 2011 after a valve in the Integrated Power Package (IPP) of F-35A test aircraft AF-4 failed. On 18 August 2011 the flight ban was lifted to allow monitored operations. A permanent resolution would be installed later.
January 2012 (ejection seat): 15 Lockheed Martin F-35s are grounded for about 12 days to repack improperly installed parachutes (reversed 180 degrees from design). The grounded aircraft are equipped with new versions of the Martin Baker US16E ejection seat, designated as -21 and -23.
January 2013 (engine, fueldraulic line): The F-35B STOVL variant was grounded Jan 18, 2013 after detection of a failure of a fueldraulic line in the aircraft’s propulsion system. The Pentagon cleared all 25 F-35B aircraft to resume flight tests on February 12, 2013. Problem caused by a manufacturing quality problem (wrongly crimped fuel line).
February 2013 (engine, crack 3rd stage): On Feb. 21, 2013, the Pentagon grounded all F-35 aircraft, after a routine check at Edwards Air Force Base revealed a crack in a low pressure turbine blade in the engine of an F-35A.
June 2014 (engine, oil inlet line): Fleet-wide grounding order was issued on June 13, 2014 for several days after in-flight emergency of F-35B at MCAS Yuma after major oil leak. Root cause: separated oil inlet line from the oil flow management valve.
June 2014 (engine) (current): Fleet-wide grounding from July 4, 2014 after F-35A engine fire at AFB Eglin during take-off on June 23, 2014. Root cause unknown at time of writing (July 9, 2014).
Investigation focused on third stage turbine of the F135 engine.
(EDITOR’S NOTE: And still no explanation has been provided to date by Lockheed Martin or the F-35 Joint Program Office about the mysterious incident at Lubbock Texas.
On March 11, 2013 aircraft AF 23, a US Air Force F-35A en route from Fort Worth to Nellis AFB, diverted to the commercial airport in Lubbock, Texas, after a “warning light” came on.
The aircraft remained at Lubbock for four weeks, while Lockheed personnel performed unspecified work, and finally was able to fly out on April 8.
At the Paris air show in June 2013, Lockheed spokesman Michael J. Rein told Defense-Aerospace.com that JPO had issued a statement on the incident, but this is not the case.
The mystery continues.)