Aging Aircraft: USAF F-15 Fleet Grounded

May 26, 2009 12:49 UTC by Defense Industry Daily staff

F-15C over DC
(click to view full)

“Array of Aging American Aircraft Attracting Attention” discusses the issues that accompany an air force whose fighters have an average age of over 23.5 years – vs. an average of 8.5 years in 1967. One of the most obvious consequences is the potential for fleet groundings due to unforseen structural issues caused by time and fatigue. That very fear is responsible for the #1 priority placed on bringing new KC-X aerial tankers into the fleet to complement the USA’s 1960s-era KC-135 Stratotankers.

It can also affect the fighter fleet more directly.

Following the crash of a Missouri Air National Guard F-15C aircraft Nov 2/07 (see crash simulation), the US Air Force suspended non-mission critical F-15 flight operations on Nov 3/07. While the cause of that accident is still under investigation, preliminary findings indicate that a structural failure during flight may have been responsible. In response, Japan suspended its own F-15 flights, which left them in a bit of a bind – even as Israel’s F-15s joined them on the tarmac. As the effects continue to spread and the USAF and others continue to comment on this situation, DID continues to expand its coverage of this bellwether event. A conditional restoration of the American F-15A-D fleet to flight status was soon overturned by the re-grounding of that fleet as a result of the report’s conclusions – a status that remains only been partially lifted. Meanwhile, the accident report has been released (compete with video dramatization) and the status of the remaining aircraft will have significant implications for the USAF’s future F-15 fleet size. Not to mention its other procurement programs.

Then, too, this is America. Now there’s a lawsuit.

The F-15A reached initial operational capability for the US Air Force in September 1975, and approximately 670 F-15s remain in the USAF’s inventory. Current F-15 flying locations include bases in the continental United States, Alaska, England, Hawaii, Japan and the Middle East, and the aircraft are active on the Iraqi and Afghan fronts. The Missouri Air National Guard F-15C that crashed was built in 1980.

Lt. Gen. Gary L. North, US CENTCOM Combined Forces Air Component commander, is maintaining the newer F-15E Strike Eagles on ground alert, to be used if required. Otherwise, he says he will accomplish all assigned missions using a variety of fighter, attack and bomber aircraft, and unmanned aerial vehicles. Lt. Gen. North added that:

“I worry about the health of our aging fleet and how sometimes it is not well understood by those our Airmen protect… The investigation will get to the cause of the accident.”

USAF Chief of Staff Michael Moseley was even more specific in an Oct 30/07 interview with GovExec.com:

“The F-15s and F-16s were designed and built in the late ’60s and ’70s. Some of them were produced up until the early ’80s. But they’ve led a pretty hard life of 17 years of combat. So you have to replace them with something, because we were continuing to restrict the airplanes. In the F-15 case, we’ve got the airplane restricted to 1.5 Mach. It was designed to be a 2.5 Mach airplane. We’ve got it limited on maneuvering restrictions because we’ve had tail cracks, fuselage cracks, cracks in the wings. The problem with that is – and Mike Wynne uses this analogy – it’s almost like going to the Indy 500 race practicing all the way up until Memorial Day at 60 miles an hour, and then on game day, accelerating the car out to 200 miles an hour. It’s not the time to be doing that on game day.

So in our training models and in our scenarios, we’re limiting these airplanes because they’re restricted and getting old. So there’s two parts to the recapitalization of the fighter inventory. The first part is the existing stuff is old and it’s getting broke, and it’s getting harder to get it out of depot on time. And our availability rates and our in-commission rates are going down. The ability to generate the sorties on those old airplanes is in the wrong direction.”

And Flight International:

“A USAF F-15 crashed in the Gulf of Mexico in 2002 when it broke up after the leading edge of its left vertical stabiliser detached in a high-speed dive to Mach 1.97. The pilot was killed.

The USAF says it began replacing the leading edge and upper aft portion of the vertical stabilisers during depot overhaul and has so far completed 463 of its 664 aircraft. The F-15 involved in the Missouri accident had its vertical stabilisers repaired in August 2003, the service says.”

Further investigation focused on the plane’s longerons, which connect the aircraft’s metal ‘skin’ to the frame, and run along the length and side of the aircraft. Both the Accident Investigation Board and Boeing simulations have indicated them as a possible source of catastrophic failure; indeed, DID had wondered why structural failure was suspected immediately, and it with that revelation it began to make sense. As DID explained at the time, if one or more of those longerons had failed, the stresses on the airframe could have folded or broken the plane in half – a very unusual form of accident. Eventually, the publication of the formal report confirmed that hypothesis:

“The one longeron, already not up to design specifications, cracked apart under the stress of a 7G turn, the colonel said. This led to the other longerons failing as well, which then caused the cockpit to separate from the rest of the fuselage. The pilot was able to eject, but suffered a broken arm when the canopy snapped off.”

Nor is this problem confined to the USA – or even to the here and now.

The Chinese government’s Xinhua agency reports that Japan has also grounded its F-15 fleet. Japan’s F-15Js were built locally under license, on a more recent production schedule, but their oldest planes do date back to 1980. This is a precautionary measure until more is known.

Since Japan’s F-16-derived F-2 fighters are also grounded in the wake of a recent crash at Nagoya, this leaves 1960s era F-4EJ ‘Kai’ Phantom IIs as Japan’s interceptor and fighter patrol fleet for the time being.

Israel confirmed to Flight International that it had also grounded its 70 F-15A-D air superiority aircraft, which are undergoing multi-role conversions, and its F-15I Strike Eagles. The Strike Eagles were later removed from the USA’s concern list, but its F-15 A-D fleet is an important component of Israeli air defenses alongside its larger F-16 fleet.

Gen. John D.W. Corley, the commander of US Air Combat Command, was not encouraged by the results of the report, and of the in-depth fleet inspections that led to 40% of the Eagle fleet remaining on the ground over 3 months after the investigation:

“The difficulty is that issues have been found with F-15s built between 1978 and 1985, across A through D models at several bases, so no one source of the problem can be isolated… This isn’t just about one pilot in one aircraft with one bad part… I have a fleet that is 100 percent fatigued, and 40 percent of that has bad parts. The long-term future of the F-15 is in question… We don’t have a full and healthy fleet, so we’ve gotten behind on training missions, instructor certifications, classes and exercises…

We’re going over each and every aircraft to make a determination. We will take some F-15s out of the inventory. It just doesn’t make sense to spend the time and money if it won’t be worth it for some aircraft.”

Updates

May 26/09: Aviation Week reports that the USAF is looking into the possibility of a Service Life Extension Program for its F-15A-D fleet, designed to increase their service lives from 8,000 flight hours to 12,000.

The move is driven, in part, by the impending collapse of Air National Guard wings that can be used in domestic air sovereignty patrols, as older fighters retire and are not replaced. The USAF is accelerating the retirement of 250 F-16 and F-15 fighters in FY 2010, and current plans calls for 2 ANG air sovereignty mission units to get F-22s, 4 to get receive upgraded F-15A-Ds, and the remaining 12 are yet to be determined.

March 22/08: Maj. Stephen Stilwell, a pilot for Southwest Airlines whose Missouri Air National Guard F-15C’s mid-air crackup began the fleet groundings, has filed suit in U.S. District Court against claiming Boeing Corp. His injuries left him with a 10-inch metal plate in the injured arm and shoulder, and he reports that he has suffered from chronic pain since the accident.

Stilwell’s suit, filed by attorney Morry S. Cole, says that Boeing knew or should have know that the F-15 as manufactured allowed and permitted for catastrophic flight break-up, and adds that Boeing failed to notify the Air Force and Missouri Air National Guard of “the likelihood of excess stress concentrations, fatigue cracking, structural failure and in-flight aircraft break up as a result of the structural deficiencies.” St. Louis Post-Dispatch.

February 2008: The largest effects of the F-15 fleet’s grounding may yet play out on the procurement front. If many of the USAF’s F-15s, which were supposed to serve until 2025 or so, must be retired, how should they be replaced? Read “Aging F-15s: Ripples Hitting the F-22, F-35 Programs.”

Jan 21/08: This week’s edition of the “Today’s Air Force” show highlights how the Air Force carried on its mission while more than 700 of its F-15 Eagles were grounded. See “The Eagle flies once again!” on the Pentagon Channel, American Forces Radio and Television Service stations around the world, and video podcast [30 minutes].

Jan 14/08: Officials begin flight operations again as 39 of the 18th Wing F-15C/Ds at Kadena Air Base, Japan are cleared to fly again after remaining on the ground for more than 2 months as a result of a fleet-wide stand-down. See USAF story.

Jan 10/08: According to the Air Combat Command Accident Investigation Board report released on this day. Their conclusion? The plane was simply too old:

“…a technical analysis of the recovered F-15C wreckage determined that the longeron didn’t meet blueprint specifications. This defect led to a series of fatigue cracks in the right upper longeron. These cracks expanded under life cycle stress, causing the longeron to fail, which initiated a catastrophic failure of the remaining support structures and led to the aircraft breaking apart in flight… the pilot’s actions during the mishap sequence were focused, precise and appropriate. The pilot’s actions did not contribute to the mishap, said Colonel Wignall. In addition, a thorough review of local maintenance procedures revealed no problems or adverse trends which could have contributed to the accident.”

Col. William Wignall, the head of the accident investigation added that:

“We’ve had great involvement from Boeing during the investigation. In fact, they’re the ones who determined the longeron was the problem. This was then confirmed by the Air Force Research Laboratory.”

See the USAF’s “F-15 Eagle accident report released,” and the accompanying video dramatization, as well as “Air Force leaders discuss F-15 accident, future.”

Jan 9/08: Air Combat Command officials clear 60% of the F-15A-D fleet for flying status, and recommends a limited return to flight for those planes that have cleared all inspections. The decision follows detailed information briefed on Jan 4/08 to Air Combat Command from the Air Force’s F-15 systems program manager, senior engineers from Boeing and the Warner Robins Air Logistics Center; as well as a briefing received on Jan 9/08 from the Accident Investigation Board president.

The USAF report describes inspections as “more than 90% complete,” with remaining inspections focusing primarily on the forward longerons. Thus far, 9 other F-15s have been found with longeron fatigue-cracks, and almost 40% of inspected aircraft have at least 1 longeron that is thinner than blueprint specifications. ACC believes each affected F-15 will have to be analyzed to determine if there is sufficient strength in the non-specification longeron, and this analysis will take place at the Warner-Robbins Air Logistics Center over the next 4 weeks. A number of F-15s are scheduled to be retired in 2009, and calculating the cost of fixes and airframe life of fixed aircraft could have a substantial bearing on the size of the USAF’s future F-15 fleet.

Meanwhile, the 2-month grounding, which has been the longest of any USAF jet fighter, is a gift that keeps on giving. Fully 75% of US Air Force and Air National Guard F-15A-D pilots have lost their currency status for solo flight, and another week would have made it 100%. Instructor pilots have retained their currency and will begin flying F-15B/Ds with the other pilots, so the pilots can land the plane and regain their status. This will be followed by further pilot training, which is required to regain operational proficiency status. USAF report | Flight International.

Dec 27/07: The Associated Press details some of the ripple effects created by the F-15 A-D grounding. With the F-15s in Massachusetts out of commission, the Vermont Air National Guard (ANG) is covering the whole Northeast. The Oregon ANG’s fighters are grounded, so the California Air National Guard is standing watch for the entire West Coast plus slices of Arizona and Nevada. To meet that need, the Fresno, CA based 144th Fighter Wing has had to borrow F-16s from bases in Indiana and Arizona and trim back training.

The Minnesota ANG is manning sites in Hawaii, while the Illinois ANG covers Louisiana. In Alaska, the new F-22 Raptors are stepping in – and so are Canadian CF-18s, which have intercepted several Russian bombers near Alaska in recent weeks.

Dec 10/07: The F-15 A-Ds remain grounded. A USAF update informs us that throughout the Air Force, maintainers have found cracks in the upper longerons of 8 F-15s so far: 4 from Air National Guard 173rd Fighter Wing, Kingsley Field, OR; 2 from USAF 18th Wing, Kadena Air Base, Japan; 1 from 325th Fighter Wing, Tyndall AFB, FL; and 1 from ANG 131st Fighter Wing, St. Louis, MO.

Inspections are underway using previous methods, until the Warner Robins ALC develops new ones for the fleet. After the area’s paint is stripped and bare metal is exposed, Airmen apply chemicals that reveal cracks under a black light. “Other inspections in hard-to-see areas are done with a boar scope [sic... maybe they mean "borescope"?] – a tool that uses a tiny camera and fits in tight areas.” Inspection time per aircraft is 12.5 to over 20 hours, and the 2-seat B and D models are more time consuming because the rear seat must be removed to access the upper longerons. USAF story.

UPDATE from USAF: “Yes, other readers pointed that out as well (although yours was the funniest). The story was corrected…”

Dec 3/07: It’s now official. Gen. John D.W. Corley, the commander of Air Combat Command orders the stand-down of all ACC F-15 A-Ds until further notice, and recommends the same for all other branches of the USAF. The stand-down does not affect the F-15E Strike Eagle and its variants abroad.

Technical experts with the Warner Robins Air Logistics Center at Robins Air Force Base, GA are developing a specific inspection technique for the suspect area, based on the recent findings. However, unlike previous inspections, the inspected aircraft will not be returned to flight until the F-15 A-D model findings and data have been analyzed, required inspections have been accomplished, and the necessary repair or mitigation actions have been completed. To date, longeron cracks have been discovered in an additional 4 aircraft. USAF release.

Nov 28/07: The accident investigation board (AIB) report leads to the recommended re-grounding of the USAF F-15 A-D fleet, and almost certainly those of other countries as well. The new AIB findings have drawn attention to the F-15′s upper longerons near the canopy of the aircraft, which appear to have cracked and failed. Longerons connect the aircraft’s metal ‘skin’ to the frame, and run along the length and side of the aircraft. In addition to the AIB’s conclusions, manufacturer simulations have indicated that a catastrophic failure could result from such cracks, which were also discovered along the same longeron area during 2 recent inspections of F-15C aircraft.

The commander of Air Combat Command has recommended the stand-down of all F-15 A-D model aircraft across the US military, and ordered a renewed fleet-wide inspection of all ACC F-15 A-D model aircraft using a very specific inspection technique for the suspect area. The multi-role 2-seat F-15E Strike Eagles, which were manufactured later and had several design changes made, remain exempt from these cautions and exceptions. USAF article.

Nov 21/07: All USAF’s F-15s are being returned to flight status, despite acknowledgment that the service is accepting a degree of risk in doing so. Gen. John D.W. Corley, commander, Air Combat Command:

“The cause of the mishap remains under investigation… At the same time, structural engineers have conducted in-depth technical reviews of data from multiple sources… First, we focused on the F-15Es. They are… structurally different than the A-D models. Problems identified during years of A-D model usage were designed “out” of the E-model… Next, we concentrated on the remainder of the grounded fleet. The AIB(Accident Investigation Board) is now focused on the area just aft of the cockpit and slightly forward of the inlets. Warner Robins ALC mandated a thorough inspection and repair of all structural components in this area. I have directed each F-15 aircraft be inspected and cleared before returning to operational status. Today, ACC issued (a flight crew information file) and Warner Robins ALC issued an Operational Supplemental Tech Order to further direct and guide your pre-flight and post-flight actions.”

There are 666 F-15s in the Air Force inventory. As of this day, 219 of the 224 E-models and 294 of the 442 A-D models in the USAF’s inventory have been inspected and re-cleared for flight.

Nov 19/07: Shortly after becoming the first deployed F-15E unit in the Air Force to return to full operational capability following the Air Force’s fleet-wide grounding of the aircraft, the 455th Expeditionary Maintenance Squadron at Bagram AFB, Afghanistan, began the move from 5-7 day phase inspections every 200 flight hours, to a phase inspection every 400 flight hours. This change isn’t slated for implementation until 2008, but it’s being implemented early at Bagram AFB to keep more F-15Es in the air and meet mission demands.

The USAF says that its engineers at the Warner-Robins Air Force Base Air Logistics Center, GA looked carefully at all the data after years of F-15E analysis and testing, before approving the change. USAF release.

Nov 15/07: A USAF release says that an order issued by Air Combat Command’s Commander Gen. John Corley on Nov 11/07 mandates a 13-hour Time-Compliance Technical Order (TCTO) on location for each of the USAF’s F-15E Strike Eagles, to inspect hydraulic system lines, the fuselage structure, and structure-related panels. Aircraft that pass this inspection may return to flight status, and similar procedures are likely to be underway for Israel’s F-15Is. ACC Combat Aircraft Division chief Col. Frederick Jones said that this was possible because:

“We were able to determine, based on initial reports from an engineering analysis, that the F-15E is not susceptible to the same potential cause of the Missouri mishap.”

The TCTO inspection is designed to confirm the engineering analysis, and aircraft deployed the CENTCOM has apparently completed inspections and returned to flying status. This still leaves 2/3 of the USAF’s F-15 fleet grounded, however, as the F-15A-D models remain under suspicion. The F-15Es are about 15 years old on average, but the F-15A-D models were introduced earlier. Maj. Gen. David Gillett, ACC director of Logistics said that:

“What we’ve got here is an example in the C model of what happens when you have an airplane that’s about 25 years old… What you find is that it becomes more and more expensive to modify [the F-15 airframe] over time… Our costs have gone up 87 percent in the last five years and continue to rise rapidly. Even when you invest in an old airframe – you still have an old airframe.”

Additional Readings & Sources

• Gannett’s Military Times – F-15′s midair breakup. Hosts the Pentagon’s Crash Simulation.

• DID Spotlight article – Aging Array of American Aircraft Attracting Attention. The problem is not at all unique to America.

• Reuters (Oct 15/08) – US Air Force eyes fighter cuts to boost modernization. The force cuts would reduce the reserve fleets by about 137 F-15s, but it would also cut 177 F-16 fighters and 9 A-10s. The goal is to save $3.4 billion, to be applied to F-35 purchases.

• Investors Business Daily (Dec 7/07) – The Eagles are Grounded. Quotes ACC chief Maj. Gen. Ronald Keys re: the slippage of fighter fleet average age from 1967 to the present.

• Flight International (Nov 6/07) – VIDEO: Israel and Japan join US in grounding F-15s

• USAF (Nov 6/07) – Mission continues during grounding of F-15s.

• US DoD DefenseLINK (Nov 6/07) – Air Force Grounds F-15 Fleet Following Crash

• Xinhua, via Military.com – Now Japan Grounds F-15 Fighters

• USAF (Nov 4/07) – Air Force suspends some F-15 operations