Heavy use for close air support in Afghanistan meant extended loiter times and flying at high altitudes with wings swept forward, prematurely aging critical wing components. Master Sgt. William Greer
Photo Caption & Credits

Repairing Broken Bones

Jan. 25, 2021

A smaller fleet, fatigue tests and continuous depot repairs should keep B-1s flying until the B-21 joins the force. 

The Air Force expects to have enough money, spare parts, and maintainers to keep its B-1B bombers flying safely until they are replaced by the B-21—due largely to Congress allowing USAF to retire 17 of the most problem-prone Bones. But the service is well behind on structural fatigue tests meant to better understand life-limiting cracks and stress on B-1 airframes, so there could be surprises ahead.   

The 2021 National Defense Authorization Act clears the way for the Air Force to reduce the B-1 fleet from 62 aircraft to 45. If the B-21 delivers on schedule, enough should be on hand by 2030 to 2036 to retire the rest of the B-1s. 

The Air Force has pushed since September 2019 to shed its most-worn B-1s, saying it would use the savings to keep the remainder of the fleet ready for action. Parts shortages and structural and systematic problems have driven the B-1’s mission capable numbers to single digits in recent years.  

Engineering assessments found “17 of the B-1Bs were in a state that would require tens of millions of dollars per aircraft” to get them to a baseline capability, said a spokesman for Air Force Global Strike Command (AFGSC). By retiring these airplanes in 2021 “maintenance dollars and manpower can be focused on the healthiest aircraft in the fleet,” he said. While other fleet sizes were considered, keeping 45 aircraft ensures enough planes to meet “global commitments.”

It is a cost avoidance. It does help … our healthiest aircraft to [retire] … these 17.

Bomber Program Executive Officer Brig. Gen. John Newberry

The B-1s average more than 32 years old. “This is the first step in gradually drawing down the fleet,” he said, to make way for the B-21.

The Air Force retired 33 of its B-1s in 2003, leaving 60 aircraft, on the argument that a severe maintenance backlog of the fleet would cost $2 billion to fix. The Air Force said then it made no financial sense to keep all those aircraft in service and, once the fleet was cut, mission capable rates soared. Ironically, that persuaded field commanders to call on them more—which soon wore down their newfound reliability. Congress later ordered seven of the bombers brought back to service.

Retiring 17 more planes now will leave enough resources for B-1 repairs and structural modifications to “get us as long as we need for the B-1 to fly,” bomber Program Executive Officer Brig. Gen. John P. Newberry told Air Force Magazine in a December interview.

B-1
© Mike Badrocke

“It obviously helps the Air Force to get 17 aircraft off the books,” he explained. “They will be the most expensive to sustain, right? … So, it is a cost avoidance. It does help … the 45 of our healthiest aircraft to not [fix] these 17 aircraft.”

AFGSC hasn’t said yet when the 17 bombers will go to the boneyard, but Congress stipulated that the aircraft must be kept in Type 2000 “or better” storage, meaning they’ll be available for harvesting of parts, but not recallable to active service. The 17 airplanes will come about evenly from the two B-1 bases: Dyess Air Force Base, Texas, and Ellsworth Air Force Base, S.D.

A number of think tanks, including the Air Force Association’s Mitchell Institute for Aerospace Studies, have urged the Air Force not to retire the B-1s and to retain as many bombers as possible until the B-21 is operating in significant numbers. Indeed, the Air Force’s new goal is to have a fleet of 220 bombers—far more than the 135 it will have once the 17 B-1s are gone. 

The B-1 was designed to fly 8,000 to 10,000 hours, or about 30 years in normal service, depending on the rate of usage, but the fleet has actually flown about 12,000 hours, on average. To help predict where physical failures are likely to occur, the Air Force has, since 2012, run a structural fatigue test on a B-1 carcass and wing taken out of the boneyard at Davis-Monthan Air Force Base, Ariz. In a test apparatus at Boeing’s facilities near Seattle, a series of bars and pulleys apply forces on the wing and fuselage, simulating the effects of multiple flights. This ages the test article faster than the operational fleet and helps engineers discover structural vulnerabilities before they appear in operational aircraft.

The goal, Newberry said, is to achieve 28,000 simulated flight hours on the representative wing, and 27,000 hours on the fuselage; what’s called the “durable life” of the airplane. So far, tests have simulated 15,875 hours on the wing, but just 7,154 hours on the fuselage.

Those hours are actually half of the real numbers, though, because fatigue tests are meant to establish a “Certified Structural Life.” The service is only comfortable flying the fleet to half the number of simulated hours applied to the fatigue test article, to leave a generous margin for the unexpected.  Consequently, while the wing taken from the boneyard had 3,085 hours already on it, the structural fatigue test could only be credited 1,547 hours at the outset. 

“Unfortunately, we’re behind,” Newberry allowed. “The [actual] fleet average is a little over 12,000 [hours].” The wing test article is ahead of the fleet average, but the fuselage lags it.  

The reason the test is behind has to do with the way structural fatigue tests are run. If a structural failure occurs on the test article, the test must be stopped while engineers figure out what broke, and why. A repair is then designed, prototyped, and installed. The fatigue test then continues. When the operational fleet encounters the same problem, a fix is ready to go. Or, it may be installed preventively during depot maintenance.

Major problems that required stopping the test included “on the wing … leading and trailing tabs, upper wing splice bolts, and drain holes,” Newberry said. On the fuselage, it was “cracks … on the dorsal and shoulder longerons.” A repair is now being prototyped on the forward intermediate fuselage. There are also rib cracks, he said, as well as “shearing bolts and tension clips. So there are various items.”

“We want to get ahead of the fleet,” Newberry said, he can’t predict when that will happen, because it’s impossible to know when the next failure will occur. Even now, the test is on hold while a fix to a longeron—a main structural piece that carries heavy loads in the structure—is being prototyped. 

Sheet metal and aircraft technicians check wing components on the B-1 Structures Repair Line at Tinker Air Force Base, Okla. The Air Force anticipates cycling five to eight B-1s through Tinker annually, with each aircraft returning for further work every five years or so, until the B-1s are retired in the 2030s. Kelly White/USAF

Unintended Consequences

The B-1 was used hard in Afghanistan and Iraq as a high-speed close air support (CAS) platform. Its wings swept forward at high altitude, it would loiter, waiting for a call to help troops in trouble, then swoop down at high speed. But it was designed instead for the strategic mission: low-altitude penetration missions with wings swept back. The CAS mission put undue stress on the swing-wing mechanism and attach points. 

“Now we’re having to pay the piper,” then-Air Force Chief of Staff Gen. David L. Goldfein said in 2019. 

The Air Force no longer intends to use the B-1s for close air support. Throughout 2020, the Bone has been limited to bomber task force missions, conducting short-notice, short-duration deployments to Europe, the Pacific, and the Middle East.

Air Force Materiel Command (AFMC) has moved quickly to restore the B-1’s mission capable rates to percentages above 60 percent, far better than the single-digit performance of just a few years ago.

At the B-1 depot maintenance center at Tinker Air Force Base, Okla., a separate “speed line” was set up to repair specific problems crushing the B-1s readiness, such as the ejection system, cracks in the forward intermediate fuselage, and wing problems. Spare parts, chronically in short supply, also got a boost in funding.

Availability still suffered, though, as compounding structural problems overwhelmed maintainers, according to USAF B-1 program manager William Barnes.

“When we were at very low mission capability rates … we were behind in analyzing some of the data we were seeing in the full-scale fatigue test,” Barnes said. As the data were analyzed, it became clear that the airframe was aging fast, requiring more invasive and time-consuming inspections.

“A lot of that work fell on the backs of the maintainers out in the field,” he said. There wasn’t enough capacity at the depot, “nor did we have all the supplies we needed to perform that repair work.”

In May 2018, a B-1’s ejection system failed during an in-flight emergency. The crew landed safely, but the incident prompted a new wave of frequent fleetwide B-1 inspections, Barnes said.

“Since that time, we got the ejection system all fixed and got that work off the field maintainers,” Barnes said. “We brought a lot of the repair work into the depot … and then in 2019 … stood up the dedicated repair line at Tinker … where we bring in aircraft for nothing but structural repairs.” It’s, “to my knowledge, … the only dedicated structural repair line in the Air Force.” Doing it that way allowed USAF to hire extra workers for the surge repairs “and let the maintainers get back to their day-to-day job of launching aircraft and meeting mission needs.”

All that “significantly improved the availability of the fleet,” Barnes said. 

“The B-1 has never been grounded due to structural problems,” said Maj. Gen Mark E. Weatherington, commander of 8th Air Force and the Joint Global Strike Operations Center,  in a statement provided to Air Force Magazine.

While Gen. Timothy M. Ray, commander of Global Strike Command, “directed a B-1 safety stand-down in March 2019 to address a Time Compliance Technical Order inspection of the B-1 drogue chute system,” he noted, the fleet was flying again the following month. 

“In the past six months we have seen an impressive 71 percent increase in bomber training readiness,” Weatherington noted. In 2020, he pointed out, B-1 detachments conducted six bomber task force (BTF) deployments and “eight BTF long-range strike missions from home station; a significant achievement.” The B-1 “improved in both readiness and maintenance rates while executing BTF missions in support of U.S. Strategic Command and various geographic commands.” The B-1 is “combat-ready and can employ weapons anywhere in the world” in support of the U.S., its allies, and partners, he said.

But big repairs remain to be done. A longeron replacement in the forward intermediate fuselage that will require substantial disassembly of the airplane will be an intensive process. 

“We are going to bring an aircraft into Boeing’s Palmdale [Calif.,] facility in April to perform a prototype of that repair,” Barnes said, “where we will replace the forward intermediate fuselage, and we will repair the ‘shoulder’ longeron,” and this will  be the prototype for a fleetwide fix. The repair apparatus will be brought to the depot for the fleet install, “but that’s not going to happen until the ’23-’24 time frame,” he said. “So, we’re still a few years away from where we have to be.”

Lt. Col. Joseph Lay, material leader for B-1 engineering, said the longeron is “a huge section of the backbone of the aircraft.” Doing the forward fuselage and the longeron at the same time “facilitate each other” because they require access to the same areas. 

The prototype will be done on a B-1 already headed into depot, and it will have “an extended stay” in that status, Newberry noted. 

“It’ll go to Tinker for a little bit,” then to Boeing’s Palmdale, facility, “and then go back … and finish out its normal depot. It was picked because it was ready for the depot, and it was  … needing a forward intermediate fuselage replace as well.”

Since dropping their close air support mission, B-1B bombers have seen increased mission capable rates, enabling them to participate in bomber task force missions and, here, in Valiant Shield 2020, a joint force exercise in the Indo-Pacific.
Lance Cpl. Tyler Harmon/USMC

Digital Twinning 

In October 2020, Air Force Materiel Command announced a partnership with Wichita State University to create a “digital twin” of the B-1. In a press release, Lay said the digital twin will help USAF anticipate cracks and develop repairs so that when needed, they are “on the shelf” and ready to go. Another B-1 carcass from the boneyard was trucked to Wichita, Kans., where every part is being disassembled, inspected, and digitally scanned. This will allow USAF to “discover all the places that saw structural failure or damage,” creating a maintenance overlay that will be a “living medical record” for the airplane, Lay said. The process of scanning the aircraft down to every bolt will take about six years.  

With a fleet of 45 B-1s, the aircraft will flow through the depot at about nine per year—down from 11 or 12 with the larger fleet—so it will take five years to cycle the whole fleet through, Newberry said. Before the B-1’s putative wholesale retirement, each airplane will likely go through at least two more depot visits. In addition, Barnes said, there will typically be “two to four” B-1s in the structural repair line, meaning “five to eight” B-1s at Tinker “at any given time.” 

Ray has also said that eight B-1s will be dedicated to testing a range of new weapons. So, assuming an average of seven Bones in depot on average, the operational fleet will number about 30 deployable aircraft.

Among other munitions, the B-1s will be testing hypersonic missiles, expected to be carried on external hardpoints not used since before the START treaty dictated that they be closed up. AFMC officials said this shouldn’t create any new structural stresses on the airplane.

“The hypersonics testing right now has no impact to our full- scale fatigue testing,” Newberry said, nor do the new weapons require any changes—wing fences, vortex generators, etc.—to the outer shape of the airplane. When new weapons are introduced, “typically, we adjust the weapon, not the platform.”

Continuing the full-scale fatigue test right up until the B-1’s retirement is a good investment, he said. It allows the Air Force “insight to the structural health” of the bomber, and a preview of big repairs needed, so Tinker can be “prepared for it, with parts and such.” 

Given the stress test, digital twin and a surveillance program that tracks the stresses encountered by each B-1 on each flight, the fleet is “safer today than it was yesterday, and we [plan to] make it even safer,” Newberry said. 

“As we continue with the full-scale fatigue test article, we learn more,” he added. “We find other, very small or very significant structural repairs that need to happen.” Structure repairs will continue “probably for the remaining life of the aircraft, Newberry said. 

“There’s a lot of work ahead of us, but we’re on a great trajectory from where we were in the past.”