Forum: F-16 Design & Construction

LinkF16SimDude wrote:

The metallurgical fix was doin' something to the shaft call "re-peening". Haven't a clue what that is....

I've not heard of "re-peening" - Maybe they "shot peened" the part twice? Below is the definition of shot peening from Wikipedia.

Many compressor blades are also "laser peened" where the same process is used, but water is passed over the blade while it is "blasted" by a high power laser. It gives deeper penetration to the "stress layer" but is VERY expensive from what I know. FOD tolerant compressor blades often use this method to make them stronger.

Shot peening is a process used to produce a compressive residual stress layer and modify mechanical properties of metals. It entails impacting a surface with shot (round metallic, glass or ceramic particles) with force sufficient to create plastic deformation. It is similar to sandblasting, except that it operates by the mechanism of plasticity rather than abrasion: each particle functions as a ball-peen hammer. In practice, this means that less material is removed by the process, and less dust created.

Peening a surface spreads it plastically in the manner of a rivet, causing changes in the mechanical properties of the surface. Shot peening is often called for in aircraft repairs to relieve tensile stresses built up in the grinding process and replace them with beneficial compressive stresses. Usually, peening can increase life-time of parts up to 15%.

Shot peening may be used for cosmetic effect. The surface roughness resulting from the overlapping dimples causes light to scatter upon reflection. Because peening typically produces larger surface features than sand-blasting, the resulting effect is more pronounced.

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LinkF16SimDude wrote:

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.

It is a surface stress-relief technique, short for re-shot-peening.

http://en.wikipedia.org/wiki/Shot_peening

oops, sorry, didn't see TEG's reply on page 2. I replied from page 1... Sad

SweetPete, they are used for electrical power generation. They are beefed up from the flight engines, due to the fact that weight is not an issue, but many of the parts are identical.

Back in 2001 we had what was first called a tower shaft failure on a -229, (the only one I have ever heard of) after it was torn apart it was determined that the gearbox had seized causing the the tower shaft to be sheared apart. I am not to sure if it was the bearings in the gearbox or something else internal but anyway it meant a complete change of the core as well as all remaining bearings and the entire oil system.

Elliboom wrote:

SweetPete, they are used for electrical power generation. They are beefed up from the flight engines, due to the fact that weight is not an issue, but many of the parts are identical.

Pratt & Whitney only developed two industrial engines: the FT4 (a derivative of the JT4), and the FT8 (a derivative of the JT8). Looking at past emails, I presume you are referring to the FT4 engine.

For those who are not familiar with it, the JT4 (from which the FT4 was derived) was also known by the military designation of J75, which flew in the F-105 and F-106 fighters. We're talking some OLD technology.

I can tell you that on an engine reliability basis, the F100 is orders of magnitude better than the J75 ever was. You may, however, have struck on a possible reason for why towershaft failures are still simulated as part of training for the F-16. This could be a legacy standard practice, from the days when our single engine fighters were not quite so reliable as they are today.

Just a thought.

tmofarrvl wrote:

Pratt & Whitney only developed two industrial engines: the FT4 (a derivative of the JT4), and the FT8 (a derivative of the JT8).

I knew PW had more than just 2 industrial/marine engines. After a little digging on the net, I've found the following details. I've tried to give the civil/military designation for the engines basis of design and it's power rating.

Pratt and Whitney’s (P&W) aeroderivative fleet includes the GG3/FT3, GG4/FT4, FT8. FT12, ST16 and other turbines, with applications including electrical power generation, offshore platform power, oil and gas transmission, industrial drives and marine propulsion. Collectively, P&W industrial turbines have accumulated hundreds of millions of service hours.

GG3/FT3 - The GG3 (“Gas Generator” 3) or FT3 (“Free Turbine” 3) is derived from Pratt & Whitney’s JT3. The JT3 was one of the first twin spooled designed turbojets dating back to the 1950’s, its military version was the J57 It develops 10,500 horsepower. This was P&W’s first entry into the industrial aeroderivative market, which currently has an installed base exceeding 1,300 units.

GG4/FT4 - P&W’s first entry into the industrial aeroderivative market was the GG4/FT4, which currently has an installed base exceeding 1,300 units. The GG4/FT4 is based off the JT4 which is known as the military's J75 engine. It was rated at 16,000 horsepower.

FT8 - The FT8 is a twin spool gas turbine introduced in 1986, derived from one of the most successful jet engines in commercial aviation history, the JT8D. The FT8 generates 35,700 shaft horsepower and is commonly packaged in a twin engine configuration, “TwinPac”, which drives a single electrical generator. A TwinPac provides ~ 73,000 shaft horsepower Twisted Evil

and 55 Mw of electricity. (The JT8D was a turbofan developed from the J52) "FT8 is currently running at fleet reliability levels greater than 99 percent and has accumulated more than 1.9 million hours of operation" - P&W

GG12/FT12 - Based off the Pratt & Whitney JT12 and J60 turbojets. The JFTD-12 (military designation T73) is a related turboshaft engine. About 4800 shaft horsepower?

ST16 and "Other Turbines" - Shrug

(anyone with details?)

Any large civil/military turbine engine company would be insane to not develop marine/industrial engines from it's aviation products. It's money in the bank! Two Cents

I know we're drifting Off Topic

but this much horsepower is WAY COOL!

In the air or on the ground, keep 'em runnin' Thumb

Salute!

Hey, engine-breath, tink the early Pratts' power was more like pounds of thrust.

Could be wrong, but my recollection of the J-57 was like 10,000 pounds in mil. And in the VooDoo, we had 17,500 pounds +/- in burner.

The twin-spool design had some problems when we pilots pushed the throttle up too fast when at a low rpm. The F-100 dudes and we F-102 folks would get what they called "chugs". If you really abused the motor, you got a no sierra compressor stall.

The VooDoo had a better intake design, and chugs were relatively unknown.

Years later, we had something like the chugs with the F-100 motor in early Vipers. Seemed to happen mostly when moving the throttle quickly in and out of burner when pulling gees. Only saw it once.

later,

Gums, you and "engine-breath" are both probably right concerning horsepower vs. thrust. Nothing says a 10,000 lb thrust aero engine can't be developed into a 10,500 hp stationary powerplant. It would require a turbine section with more stages to convert the pressure, temperature, and velocity into torque rather than a nozzle to convert them into thrust, plus other detail changes.

If your 10,000 lb thrust aero engine is moving at 1000 ft/sec (about .9 mach) it is developing 18,000 hp. To provide long term durability, Pratt could de-tune it to 10,500 hp easily. Also, the stationary engine does not have ram air from the intake, so power is reduced.

How could a pilot, sim or not, determine that he has had a towershaft failure? If anything all he would know is that he has lost all main hyd and elec power. For all he knows he could have crapped an ADG, maybe a PTO if it was violent enough. Know way to really know in a sealed cockpit 20 something feet ahead.

There were subtle differences of indications that distinguished a Tower Shaft fail from something like a Flameout or PTO Shaft fail. It's been so long that I can't recall exactly what they are. I have a couple of contacts at the Tucson sim shop. I'll see if can forward me an unclassified overview.

I'd be like to know. May help me in future troubleshooting provided the jets not a smoldering pile of metal.

SweetPete I work on FT-4's, Someone around here told me that the core of the engine was similar to the F-100, but I guess they, and now I was wrong. I am responsible for the operations and maintenance of 3 FT-4C1-LF twin pac electrical generating sites.

Okay,

This is what happens when the Tower shaft fails on the F-16 (in this case RNLAF F-16 MLU)

1. Flame-out because the towershaft normally drives the Fuel pump, and without this one the engine dies
2. RPM indicator directly moves to 0 rpm, the towershaft drives the engine alternator that provides the RPM signal.
3. Main gen failure because ADG is also not running anymore
4. Both hydraulic pumps stop working (both numbers 3 & 4 will trigger the EPU offcourse)
5. FLCS PMG will fail (also installed on ADG)

1st thing the pilot wants to do is see if he is in gliding distance of a suitable airfield, if he is his primary objective should be the FO landing. He has to pull alternate gear, and should be considering a approach-end arrestment or drag chute.

If he is not within gliding distance he could try to go for an airstart These are the things the pilot has to do then:

1. Throttle off
2. ENG CONT swiitch to SEC (this tells the engine to start in SEC)
3. Throttle idle
4. When above 20000ft maintain 275kts (to keep the rpm within airstart limits (50-25%)
5. When descending below 20000ft JFS switcht to ST II ( this will drive the Fuel pump & Alternator so that we have Fuel pressure again and ignition)
6. Then the Engine will probably start in SEC (You must start in SEC because the DEEC looks at the rpm signal coming from the Alternator, but with this start it wil be the RPM that the JFS will give to the alternator. When the DEEC looks at this the start will probably be unsuccessful)

Hope his helps a bit.

Indeed, towershaft failure was quite rare.

One Nellis Block 10 F-16A was lost near China Lake in May 1982 after suffering a towershaft failure. When the towershaft failed, it quit turning the engine gearbox, and hence quit turning the main fuel pump, ergo, the engine flamed out. Pilot didn't know all this, but started JFS to go for a restart. JFS turned the engine gearbox, which re-established main fuel pump output, and the engine restarted and ran OK. Sadly, but IAW the checklist at the time, with the engine running again, pilot turned off the JFS, so naturally the engine quit again. Next start attempt stagnated, and he jumped out OK.

P&W re-established a (recently deleted) requirement for shot peening of the tower shaft ring gear, and the failure never occurred again. But many, many EP sim rides since then made sure pilots knew how to recognize and cope with a towershaft failure, which is an extremely low probability event these days. Pretty much a waste of sim time, IMHO.

Changes after the mishap included issuance of Interim Safety Supplement 1F-16A-1SS-209 to describe use of JFS in event of towershaft geartrain failures, and how to differentiate a towershaft failure from a MFP failure.

And of course, P&W subsequently retrofitted non-shot peened tower shaft ring gears with shot peened units.

Forum: F-16 Design & Construction

Towershaft failure... what happened?