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Elite 1K Joined: Jan 31, 2004 Posts: 1563 Status: Offline

When I was a young pup doin' the Viper sim thing, at some point during every EP ride a Towershaft failure was given. Even tho the Towershaft hardly ever failed, all the studs (and later stud-ettes Wink

) were taught the procedure to address a failure.

The history of the Towershaft said it failed less than a handful of times in the early Pratt 100s then it practically never happened again. Can one our Viper historians can shed some light on what was done to make the Towershaft practically impervious to failure?

And an aux question: Since the early block Vipers shared the same engine with the Eagle, did the Eagle community have the same problems with the Towershaft or was if Viper-unique?

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are we talking about the PTO shaft?

That was my first thought to, Power Takeoff Shaft. Power rhymes with Tower could be confused other wise never heard of a tower shaft.

Enthusiast Joined: May 09, 2006 Posts: 21 Status: Offline

The tower shaft is the connection between the Compressor and the Gearbox. I've only worked 220's and never heard of any failure.

So the tower shaft is an internal part of the engine?

Yes it's internal, the tower shaft connects the engine gearbox to the core engine module. Evil or Very Mad

Learn something new everyday. Whats with the Evil or Very Mad

Mech?

Elite 1K Joined: Apr 26, 2004 Posts: 1215 Status: Offline

He's not mad sweetpete....he's from hell.....!!!!!!

It's kind of his trademark.

It's all about the icons baby! Evil or Very Mad

In the civilian world I work on an industrial version on the Pratt engine, and the early versions of this engine are also known for tower shaft failures. The towershaft is the shaft that connects the high compressor to the accessory gearbox, and with ours in the original configuration the bearings relied on drag from the gearbox to provide the proper preload when at low speeds. What would happen is at low speed the bearings balls would tend to drift and fail early. To fix this they ended up going with one of two fixes, either a double bearing configuration that had each bearing apply preload to the other, or a ceramic bearing that was much more wear resistent. Now remember this fix was on an industrial engine that was not certified for flight, so the same fix may or may not have been applied to the flight engines. And yes, since I have been here we have had 2 towershaft failures, we have soem of the very first engines of this type and they had the original configuration. Also both bearings failed right around 1200 engine running hours. Hope this helps.

What is the industrial application they are used on?

As mentioned before the tower shaft pulls power from a bevel gear on the the N2 (Core) shaft at the number 2/3 bearing area and transmits it (vertically through the intermediate case) down to the gearbox. The gearbox (in turn) runs the oil pump, fuel pump, stator generator and PTO pad. The PTO shaft then sends power into the ADG for airframe use. (See pic below)

These "handful of failures" must have been a PW-100 issue? The PW-200 engine was a modification of the -100 specifically engineered for "single engine aircraft" There were many improvements made from the -100 to increase reliability for the F-16. I'm not sure if the tower shaft was one of them of if it was limited to some bad production in early engines?

Maybe someone knows of an instance?

If one ever failed; I believe you engage the JFS so that power is "backfed" through the PTO back into the gearbox so that fore-mentioned engine accessories have power. (As when starting the engine.) This should allow the engine to limp home with some oil/fuel pressure and electrical power.

I've never heard of "tower shaft" failure on a PW-220 or PW-229.

The PTO shaft is much more likely to fail, and that is EXTREMELY rare. Very Happy

Side note: F-15s have more PTO problems than F-16s with F100 engines. Seems the F-15's AMAD/PTO has a different frequency that seems to affect the PTO bearings in the engine's gearbox more adversely than a Viper. The PW-220s and PW-229s are being improved in this area so this is no longer an issue.

Elite 1K Joined: Jan 31, 2004 Posts: 1563 Status: Offline

Engine Guy...the TS failures were indeed with the -100 Pratts. Although even after the -200 and -220Es were installed, the malfunction was still taught as part of the syllabus even tho the odds were astronomical that it would ever happen. One of those "ya never know but here's what it looks like" things.

If memory serves, the TS failures may have occured at Eddie and not in the field. The metallurgical fix was doin' something to the shaft call "re-peening". Haven't a clue what that is but there was never a failure after that.

Salute!

Funny, but back when the Earth was still cooling, our motor problems didn't seem to be related to PTO shaft failures, but other things. Guess we'll have to go to the "mishap" section or a/c database.

My understanding was that the Viper motor was, indeed, given special attention compared to the Eagle motors, as we only had one.

Seems some of our early failures were with the fuel control unit (hydromechanical, with EEC). A pilot or two tried BUC, then fried the motor. Then we may have lost another bird or two when the pilot didn't even try BUC, as they were scared.

So a few of us that had time in the T-33, with it's primitive fuel control unit, thot we'd try the BUC when still in the chocks. In fact, what we did was start the motor using BUC. Hell, it was just like starting a T-33 motor. Wait for a rumble and EGT rise, then slowly, with no reversal of the throttle lever, "push" the EGT up. If temp went up real fast, then stop and wait, then continue until the motor was smoothly idling. Turned out we could get the motor running quicker than the cosmic EEC doofer.

SIDE NOTE: The T-BIRDS used BUC when starting and running in front of the VIP's, as it diminished the squeal we normally had.

We included BUC starts on the pilot training event list ( once a month or so), and we made our studs do it when they were checking out.
********

So I don't remember PTO shaft failures as a big deal in the early days. Having flown two other jets with the J-57, I was sensitive to a PTO shaft failure. The Viper mechanization was better, as the EPU could "back feed" the fuel pump on its way to the hydraulic pumps.

later,

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Towershaft failure... what happened?