"pulling through" a propeller

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Joe Broady

Airman 1st Class
106
156
May 30, 2019
I've been reading the report on the 2015 T-28 crash that killed David Tokoph. Shortly after takeoff from Las Cruces the engine failed and the plane went down in a field. It looks like a survivable crash: no fire, wreck is upright, and cockpit appears intact. However, Tokoph sustained serious head injury and died four days later. The NTSB report notes the shoulder harness had no inertial lock. It had to be operated manually.

Internally the engine was a mess. One cylinder had to be pried off to inspect the crankcase. Metal fragments had torn up the rods, pistons, and cylinder skirts. The no. 6 rod was broken. According to the report, the crash was due to "failure of the No. 6 cylinder connecting rod, due to improper rotation of the propeller during a previous hydraulic lock."

One of the witness statements in the accident docket came from the fueler, who "observed Mr. Tokoph trying to turn the 'prop' but it was not turning." It's not clear if he was pulling the prop through or trying to crank the engine with the starter. (The statements were taken by a cop, not an accident investigator.) Tokoph asked for a ground power unit. After it was hooked up, it took three attempts to keep the engine running. The fueler said the first two attempts, the engine died about 10 seconds after the GPU was unplugged. Also, its sound was different between GPU plugged in vs. unplugged. I have a hard time understanding why external power would make a difference once the engine was running.

Another Interesting observation by the fueler was that the engine would "piss a lot of fuel" during the start attempts. Could that have been due to excessive priming draining from the blower?

The NTSB docket contains a 2000 Avweb article, "Radial Engines and Hydraulic Lock" by Rick Durden. I already knew about removing a plug from each of the lower cylinders, but I had never heard of running the engine briefly with plugs out.

NTSB accident report
accident report docket
 
Most modern (ie since late 70s) rebuilds include a "clean kit". The main component is an electric shut off valve on the bottom of the oil tank to prevent oil draining down in to the engine thereby minimising the chance of hydraulic lock. The system is such that the start circuit is disabled until the SOV is fully open.
A very small price to pay for almost completely eliminating the lock problem and all its massive costs. You must still do a pull through but will very seldom need to pull a plug.

Like you I have never heard of trying to run an engine with the lower plugs removed. Yes it would work in theory but I would not do it because often oil is trapped in the induction system and running the engine will suck that oil in in one gulp and cause a hydraulic lock that will definitely do what happened to this guy.

I would be very surprised if Tokoph's engine made rated rpm when he did his 30" check and that should have told him that his engine was too sick to fly.

As for the different sounds when the start cart was attached there are a couple of possibilities. If it was a powered generator the refueller may not have mentally separated out the generator noise. If it was a battery cart he may not have mentally sorted out the echo or sound deflection or sound reduction that the cart provided until it was pulled free.
 
Turning the engine over with one spark plug removed per cylinder (front or rear, it doesn't matter) is a classic procedure to "un-flood" an engine from extra oil or gasoline. It is particularly important in engines with inverted cylinders, such as true inverted engines such as Gipsy, or lower cylinders in radial engines, where the cause of the hydraulic lock may have remained trapped in the induction circuit behind the intake valve.

Really too bad that in this specific case the connecting rod breakage occurred too late to really prevent takeoff.
 
A deadly trap for many engines.

The big Daimler-Benz 605 did not have a hydraulic lock habit but, we always pulled it through if it had not run before on that day. There was a tendency for oil to pool in the lowest point of the supercharger boost-pipe and the German documentation lists this. An open 4mm drain union was introduced (by DB) at the lowest point of the boost pipe and always dripped oil. The documentation surprisingly listed the oil in the boost pipe as a problem because it could cause a fire inside the inlet and heat-damage the temperature probe in there.

Eng
 

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