During World War 2, my uncle was a navigator for a B-17G bomber based in the UK. On return from a bombing run over Frankfurt, the crew was forced to bail out over Belgium at 3000 feet altitude. My uncle and 3 of his crewmembers were rescued by Belgian resistance members and hidden for 8 months when Allied forces liberated western Belgium.

In the Escape and Evasion report the pilot gave on his return to 8th Army HQ, he wrote that the crew bailed out because "No 2 and 4 engines had runaway superchargers, lost oil. Would not feather with full oil pressure."

Could someone help me out here in understanding what's the likely causal sequence of events here? Why would the plane experience a "runaway supercharger?" Was that the cause of lost oil, or was it the other way around - some sort of oil loss causing loss of control of the supercharger?

Or was it that the propellers for 2 and 4 engines wouldn't feather properly (due to mechanical trouble or maybe flak damage?) causing the engine to overheat and burn oil?

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    $\begingroup$ Interesting question! Welcome to Av.SE. $\endgroup$
    – Ralph J
    May 27 at 0:58
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    $\begingroup$ and here I was envisioning a Tesla supercharger that'd grown legs... Bummer $\endgroup$
    – jwenting
    May 27 at 9:53

The B-17 uses the term "Turbo Supercharger" which refers to just a Turbocharger (As John K points out in the comments, the former is actually the technically correct term). The B-17 had an issue with runaway Turbochargers they even cover it in the original training video (start @3:14). Its effectively when the waste gate gets stuck shut and you get excessive exhaust pressure driving the turbine. Along with the high pressure it causes the engine to lean out which presents even more issues.

The excessive oil burning could be from either the excessive pressure or the increased heat as you lean out due to turbo runaway.

The regulators also had manual control so you could deliberately shut the waste gate and create the same issue.

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    $\begingroup$ Turbo supercharger is the proper technical name for any turbocharger. $\endgroup$
    – John K
    May 27 at 1:59
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    $\begingroup$ Dave, John, thank you both for these explanations and for the links to the videos. Super helpful and much appreciated. On the day my Uncle's plane went down, he was part of a group of 748 B-17s and B-24s, with fighter support from 637 P-38s, P-41s, and P-51s. Of the bombers, 23 planes were lost or were damaged beyond repair, and 359 were damaged. 7 fighter escorts were lost or damaged. Casualties were 7 KIA, 20 WIA and 203 MIA. Just on that one day. $\endgroup$
    – Steve Kaz
    May 27 at 6:03
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    $\begingroup$ Wow. Yeah a mission loss rate of 3% isn't sustainable for very long. If you wanted to survive your tour in bombers, you wanted to be in a Martin B-26, with a mission loss rate only .5%. Or a DeHavilland Mosquito which I believe was even less. My father flew C-47s in Burma supporting the assault on Mandalay. $\endgroup$
    – John K
    May 28 at 18:11

The B-17 had General Electric exhaust-gas-driven turbochargers (or more correctly, turbo-superchargers) that had pilot selectable boost levels. A selector dial in the cockpit controlled the operation of the wastegate that controls whether exhaust goes through the turbine and provide engine boost, or is bypassed directly outside. The pilot's indication of the state of boost being provided to the engines would come from the engine manifold pressure gauges.

"Runaway superchargers" would suggest that the wastegate control systems on those engines were malfunctioning, such as the waste gate failing closed and directing all the exhaust flow through the turbo, causing the turbos to over boost the engines (the "running away" part), which would cause sky-high manifold pressures, detonation, and send the cylinder head temperatures through the roof. This would necessitate a precautionary shutdown to keep them from blowing cylinders.

During the shutdown, even though they had oil pressure indication for the engines, the propellers, which operate hydraulically using engine oil, wouldn't feather. There is a feathering pump for each engine, normally electrically operated, that draws oil from somewhere in the system and can drive the prop into feather even without any engine oil pressure, as long as the pump itself has a source of oil to its suction inlet.

The turbosupercharger waste gate uses engine oil pressure for servo power for regulating the waste gate, that is also used to lubricate the turbo's bearings.

So... thinking it through, the turbo waste gate needs engine oil, and the feathering pump needs engine oil. My guess is that there were oil system problems that impacted the waste gate control systems on those two engines, causing the boost runaway, and the same problem was also starving the engine's feathering pumps so that they were unable to complete the feathering of the propellers. Maybe flak damage, or just a technical issue with the system (it would be odd for flak shrapnel to cause identical problems in two engines on opposite sides of the aircraft).

A B-17 at low altitude, well lightened, having burned off most of its fuel, dumped its bombs, and expended ammunition, would probably have been able to make it back to the UK on two engines, IF the two dead ones had feathered properly. Having two unfeathered propellers windmilling away however, is massively draggy, and would have turned it into a two-engine glider, leaving the option of a forced landing or bailing out; and it looks like they chose the latter.

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    $\begingroup$ Possibly our old friend "same guy did maintenance on both engines, and did it wrong/used wrong parts", i.e. the reason they created ETOPS rules. (Same guy can't do both engines). $\endgroup$ May 27 at 18:00
  • $\begingroup$ The problem was far more likely to be enemy flak or fighters than a mistake by a mechanic. $\endgroup$
    – rclocher3
    May 27 at 22:14
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    $\begingroup$ Even that is pretty out of the box for that sort of double failure. My guess would be a design or manufacturing defect what was relatively common. The GE turbochargers were reputed to be somewhat sensitive (for example, the turbine could not tolerate any back pressure, which is why it was exposed). $\endgroup$
    – John K
    May 28 at 0:08

This is speculative, but in diesel engines, if a turbocharger has a failed oil seal on the inlet side, the engine can then run on the oil leaking through the seal.

Since diesel engines are shut off by stopping the fuel supply, the engine then "runs away" and typically runs until it burns all it's oil, and seizes.

If these engines had a sufficiently high compression ratio, it's possible that they could auto-ignite on oil leaking into the intake.

This would support both the oil pressure issues and "runaway" description. I'm not sure if it's applicable to these engines in particular, though. Note that any engine can conceivably "diesel" if it has a sufficient compression ratio, or a fuel that ignites at a low enough temperature.


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    $\begingroup$ The B-17 bomber being asked about, its engines aren't diesel. The other answers already address the failure mode relevant here with references. $\endgroup$
    – ymb1
    May 28 at 3:47
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    $\begingroup$ You mean they're not supposed to be diesel engines @ymb1 . However, they are when they're dieseling... $\endgroup$ May 28 at 16:30
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    $\begingroup$ I was ready to sell an anecdote about some oil that spilled into a cummins turbocharger during replacement. After it was cranked, the engine ran away- Turning the key off did nothing. Everyone ran for the hills. It ended up ejecting a connecting rod through the side of the block and out of the engine bay. Was hoping to learn about an airplane doing something similar. $\endgroup$
    – Aww_Geez
    May 28 at 19:47
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    $\begingroup$ @ymb1 - The other answers covered a failure due to excessive intake boost pressure. I'm pointing out they could have failed due to unintentional dieseling and failed turbocharger seals. Any engine can be a diesel if you feed it a fuel that ignites at the chamber temperatures it achieves at maximum charge compression. $\endgroup$
    – Fake Name
    May 29 at 1:47
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    $\begingroup$ As with most aircraft, B-17 engines are low compression. I have never heard an aircraft engine dieseling. $\endgroup$ Jun 1 at 18:45

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