Kesteren – Last Thursday, 21-February-2013, the Pentagon ordered a grounding for all F-35 aircraft, after a routine check at the Edwards Air Force Base revealed a crack in a low pressure turbine blade in an engines of a F-35A. During 2007-2009 there have been repeated problems with turbine blades, which led to significant delays in the test program and a partial redesign of certain parts of the engine.
On 19-February-2013 a routine inspection took place of 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 is sent to Pratt & Whitney in Middletown (CT), USA for further investigation.
Statements JSF Program Office
The F-35 JSF Program Office said in a 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. The F-35 Joint Program Office is working closely with Pratt & Whitney and Lockheed Martin at all F-35 locations to ensure the integrity of the engine, and to return the fleet safely to flight as soon as possible.”
Chris Bogdan, head of the JSF Program Office, suggested in a briefing in Australia, that the root cause could be known before the end of this week and also when it would be something simple as a FOD (Foreign Object Damage) problem or a manufacturing glitch the F-35 could be back in the air in a couple of weeks.
Some facts about what happened
It was a F-135 engine with 700 hours, of which 409 flight hours. The aircraft was the F-35A testaircraft AF-2. The half-inch long 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. Cracks in turbine blades in the low pressure turbine section usually are caused by high thermal or other stressing loads of the turbine blades. The forces in the 40.000 lb (about 29.000 hp) engine are enormous. A grounding after such a discovery usually takes relatively short (e.g. one week), normally a manufacturing error or some incident is the root cause. Until then, for safety reasons a grounding may be the standard procedure. Reuters reported 24-Feb-2013: “In fact, two jets were airborne at air bases in Maryland and Arizona and had to be recalled, said one of the sources.” At this moment all 51 F-35s, of all versions, in use at several airfields to support the test and training program, are grounded.
Long history of engine problems since 2006
It can not 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 had been a series of engine problems with the F-135 engine.
Already in May 2006, Aviation Week reporter David A. Fulghum wrote 0a 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 4-February-2008 something similar happened to engine FTE06, also in the 3rd LPT stage, after 19 hours.
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 by 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 crackes 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, that they were convinced that the problems were solved.
In 2009, problems with redesigned engine
In July 2009, the then head of the JSF Program Office, General Heinz was still not happy with the F-135 problems, he said against 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 commissioned by the Pentagon not to comment publicly on problems with the F-135 engine.
In September 2009, again serious engine problems 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 wante to delete the second engine choice (GE / Rolls Royce F-136) a Pratt & Whitney F-135 engine broke. Again the cause seemed to lie in broken turbine blades. However, now the problem occured in the new engine type with the redesigned turbine blades.
Engine problems continueing until now
After problems in 2009 officials no longer publicly commented about the engine problem. Also there were no indications that there actually were problems with the engine or that there were any reliability issues.
In April 2011, however, Admiral Venlet, the then Head of JSF Program Office, told reporters that some engine problems were impacting on the delivery schedule.
The grounding of last week put the engine back in the spotlight of publicity. However, at this moment it is not the complex F-35B STOVL version, but an engine in an F-35A, the Air Force version.
For Pratt & Whitney, hopefully it is an one-off, and not a structural problem.
Overview of earlier F-135 engine problems 2006-2011 and sources
Aviation Week; 28-mei-2006; David Fulghum; “Joint Strike Fighter F135 Engine Burns Hotter Than Desired”
Defense Industry Daily; dec-2007; Johan Boeder; “JSF hit by serious design problems”
JSFNieuws; 8-feb-2008; “New engine problem Pratt&Whitney”
Flight International; 27-mar-2008; Graham Warwick; “F-35 JSF engines in critical tests as Congress deliberates”
Flight International; 24-nov-2008; Stephen Trimble; “Locheed delays first F-35 STOVL flight again, but engine fix”
Aviation Week; 13-sep-2009; Graham Warwick; “BREAKING NEWS: F135 damaged in ground test”
Flight International; 27-apr-2011; Stephen Trimble; “New engine snag upsets F-35 manfacturing progress”
Reuters; 24-Feb-2013; Andrea Shalal-Esa; “Half-inch crack blamed for U.S. F-35 fighter jet grounding – sources”
Overview of other F-35 groundings
May 2007: 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: On 23-Jul-2008, both flying F-35 prototypes were grounded after problems were detected with ground cooling fan electrical circuitry, DCMA reported 18-Aug-2008 that test 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 an issue due to design problems. Flights were resumed first week of September-2008.
December 2008: On 12-dec-2008 the F-35 was grounded again as a result of engine and ejection seat anomalies. Seat anomalies wered observed in ejection seat sequence during an escape system test on 20-Nov-2008. It took nearly 3 months to solve the problems and AA-1 returned to the skies 24-Feb-2009.
May 2009: The F-35 fleet didn’t fly between 7-May-2009 (84th flight of prototype AA-1) and 23-Jun-2009. No comments were available from JPO or L-M.
October 2010: F-35 fleet grounded after the fuel pump shut down above 10,000ft (3,050m).
The problem was caused by a software bug.
March 2011: The entire F-35 fleet grounded some weeks after test aircraft AF-4 experienced a dual generator failure. 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: 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 from 17-June-2011 until 23-June-2011.
August 2011: A precautionary grounding of all 20 F-35’s that had reached flying status was ordered 3-Aug-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: 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: The F-35B STOVL variant was grounded 18-Jan-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 12 February 2013. Problem caused by a manufacturing quality problem.
Februari 2013: On 21-Feb-2013, the Pentagon ordered a grounding for all F-35 aircraft, after a routine check at the Edwards Air Force Base revealed a crack in a low pressure turbine blade in an engines of a F-35A.
Author: Johan Boeder