Strange Problems Kept Bruce Bohannon From Records at Oshkosh

"It was a vibration that plagued us from the day we turbo-charged the airplane", said Bruce Bohannon, already the holder of world time-to-climb and altitude records. "The shaking was so bad that it cracked the turbo mounts and nothing we tried could stop it." The phantom vibration that haunted the Exxon Flyin' Tiger cost Bohannon 3 more World Records at Airventure 2002. "Our sponsors and fans should have seen a World Record flight, not a test flight," said Bruce.

Bohannon planned to establish a brand new record for piston engine aircraft, "Time to Climb to 12,000 Meters" (just under 40,000 feet). With the power of the Mattituck IO-555 engine and the new turbo, designed & built by Kelly Aerospace, he also planned to break his own records for "Absolute Altitude" (in class), and for "Altitude in Horizontal Flight." But there was a gremlin in the installation; one that first reared its head at Sun 'n Fun in April and finally cornered Team Tiger at Oshkosh. "We had some theories about what was causing the vibration," says Bruce, "but we hadn't figured out how to cure it."  

The Exxon Flyin' Tiger set a new record for Absolute Altitude (class c-1.b) at Sun 'n Fun, reaching 37,552 feet. During the climb, Bohannon had trouble -- the engine ran a little roughly. "I thought that I was getting lost in the leaning process as I closed the wastegate on the turbo," says Bruce. "It wasn't until we got home that I discovered that the pressurization line to the rail that feeds air to one side of the engine (3 cylinders) had come loose." The resulting air-fuel mixture imbalance explained the engine roughness; but nothing explained that infernal vibration that jarred it loose.

Still, Team Tiger had bigger fish to fry. Cooling had become a major problem since the turbo installation. CHTs were running high during the climb, and Bohannon had his sights set on a "triple play" at Oshkosh. So, after fixing the pressure line, all attention turned to engine cooling. With AirVenture just 3 months away, the crew launched a marathon mission. 

Hours of research & development, testing and modifications paid off. Engine cooling was greatly improved and Bohannon was confident he could bag the three records. "I was about to learn my lesson the hard way," he said. "We had the power, we had the cooling but we hadn't made a test flight to altitude." With little time remaining, Team Tiger left for Oshkosh with all-new turbo mounts -- but without a test flight.

July 27th, Oshkosh, Wisconsin: Fooie.

Black clouds that postponed Saturday's scheduled record attempt soon revealed a silver lining. "The weather delay bought me time to consult an expert on our mystery vibration", says Bruce. "That's the great thing about Oshkosh, so many bright minds in one place, willing to help." Bohannon asked turbo guru Bob Minnis of Avpower for his advice. "It took us 10 minutes to remove the cowling, it took Bob 10 seconds to spot the problem", says Bohannon. The culprit? The line that pressurized the fuel injector nozzles was installed in a low-pressure area. Inadequate pressurization was causing a fuel mixture problem that didn't raise red flags on the EGTs. Crewmembers moved the line to a slightly higher-pressure area pointed out by Minnis. "Bob also pointed out that the line was too small to pressurize the fuel pump, the mags and the fuel injector nozzles", says Bruce.  "We were trying to feed three accessories with one pressure line that was too small for even one of them!"

July 28th: Ready as Ever.

Sunday dawned with cautious optimism. With the patch in place, The Exxon Flyin' Tiger was "race ready" and Bohannon departed on schedule.  Crew Chief Gary Hunter (seen holding helmet at SNF 2002, right) monitored Bruce's progress as he headed for 40,000 feet. The vibration was gone and the engine was running smoother, but at 27,500 feet the good times ended. There was a sudden, drastic loss of manifold pressure. The engine ran fine by itself but Bohannon couldn't get the turbo to spool back up. He says, "My first thought was that I had blown off a pipe ahead of the firewall. I was disappointed but felt sure it was a simple fix and we'd be right back in action."

Hours later, there were no obvious answers. Team Tiger went over every inch under the cowling but found nothing wrong. "At this point, I suspected the pop-off valve on the turbo may have stuck open", says Bruce. "It was the only way I could see losing that much pressure that fast without breaking a pipe." The pop-off valve appeared to function correctly on inspection. With only one day of AirVenture left, Bohannon decided to wire it shut (just in case) and make another run.

Bruce told ANN at the time that, because of the action of the engine, he thought it was a stuck wastegate; but there was nothing he could find on the ground to bear that out. Last week, he told us, "The pop-off valve for sure did not fail. There's absolutely nothing wrong with it." That confused him at Oshkosh, too: "It was so fluid, so easy to push. Why it popped (Saturday) makes no sense." He still doesn't think there's any problem with the turbo and its wastegate. "We're not going to change a thing, until after we can duplicate the problem," he said.

The turbo, while essential to the record, really isn't stressed that hard. Ralph Benway, one of the engineers on the turbo team at Kelley Aerospace, told ANN at Sun 'n Fun, "Considering he went to 35,000 feet naturally aspirated, he really doesn't need that much boost. He would need a lot more, if he were trying for full power at 40,000 feet; but he just wants to fly there."

July 29th: Ready as it's gonna be.

Monday's record attempt brought no joy for Team Tiger. The airplane started struggling at 25,500 feet, and by 30,000' Bohannon knew there was no record in the cards. "As with most test flights, we landed with more questions than answers", said Bruce. "The folks at Kelly Aerospace have since done tests and ruled out any physical problems with the turbo and the pressure relief valve. Now they're working some real scientific questions involving possible phantom airflow problems."

When we talked with the tall altitude-hound, he called the manifestation a "strange vibration." He said, "We've got ideas of what it could be; but we don't want to speculate." Bohannon notes that complications are par for the course. "When you take a one-of-a-kind airplane and mate it with a one-of-a-kind turbocharger then try to go places piston engine planes don't normally go, it gets hairy. That's what experimental aviation is all about; learning to do what hasn't been done before."

He wanted to go home, and sort the machine out. As Donah, Bruce's wife and (as Bruce says), "the brains of the operation," told ANN, "It's much easier to do troubleshooting at home base."

Lessons learned at Oshkosh are already being applied back at Flyin' Tiger Field in Texas. Bohannon says, "We have some fuel-air mixture issues to address, starting with the improvements recommended by Bob Minnis." High-altitude flight requires pressurization of the magnetos, fuel pump and fuel injector nozzles. Minnis pointed out that the pressurization lines on The Exxon Flyin' Tiger were too small. Team Tiger is replacing them with larger lines supplying individual service to each accessory.

Bruce told us that, "What we really needed was three more weeks between Sun 'n Fun and Oshkosh." EAA: are you listening?

"We never intended to make a public test flight", says Bruce. "We have a lot of work on our desk, but when we do set another record date, we'll have already proven we can do it… several times!"

Bruce took the non-records philosophically. He had, after all, not broken anything.

"The good news is," he said, "there's a complete airplne to fly home in." The "Never Say Die" team has spoken.

FMI: exxon release, unison release