NTSB and Skydiving

I think the torque gauge reads pressure between the compressor and turbine sections of the engine, more pressure means more torque. Torque is the twisting force being applied to the propeller, so bigger engines will have more, damaged engines will have less. If you start to see that you are loosing torque, you would get worried, because something is wrong. It might only be the gauge itself, but it would still get you worried. Sorta like seeing your gas gauge empty, when you know you should be fine. Its time to pull off the freeway regardless....

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Also can someone please explain what a "rolling start" is in comparison with how you are "supposed" to take off

A rolling start is one where by the pilot rolls onto the runway, and rather than stop, apply power, and then release the brakes, they simply continue the smooth roll on to the runway into the take off. Not a bad technique in it's self, but when combined with a mid field start (i.e. rolling on to the runway at mid field) can reduce the pilot's options should the take off not go as planned. There is an old saying among old pilots: "Nothing more useless than runway behind you and altitude above you."

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and what it means to "feather an engine" vs. shut it down? Thanks ahead of time...

Without going into to much detail as to why they do this, a constant speed or variable pitch propeller (as found on Twin Otters) can have the blades positioned anywhere from flat to the wind to knife edge into the wind. When a pilot has to shut down an engine, or looses one to failure, it is prudent to "feather" or place the blade into the knife edge into the wind position to reduce drag. If this is done on a good engine, the thrust produced will drop to nearly zero.

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I have a few questions after reading many of these posts again. First, can someone please explain what "ground effect" is and what it means to come out of it...

The wingtip vortices and downwash from a wing in flight produce something called induced drag. When an airplane is flying within one wingspans height over the ground the ground itself interrupts the vortices and downwash thus reducing, or eliminating the induced drag experienced by the airframe and wing. When the aircraft climbs higher than the length of its wingspan it encounters the effects of induced drag again. If it has insufficent lift (from lifting off at to slow of an airspeed for example) it may not be able to climb or even remain in flight once outside of ground effect.

Some good reading for you:



http://en.wikipedia.org/wiki/Ground_effect_in_aircraft
http://en.wikipedia.org/wiki/Wingtip_vortices
http://en.wikipedia.org/wiki/Constant_speed_propeller

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Question for Totter, Chris or anyone else who has knowledge on this: If there was a "problem" with the torque gauge earlier that year that did not seem to be that big of a deal at the time, it seemed like there was a problem with the gauge rather than a problem with the torque... Again sorry to sound like a dumbass but can someone please explain what the torque is related to and how it is measured?

I will try to explain this as easily as possible.
Torque is basically the measure of engine power at the prop. The term torque is used because the gauge shows a value of rotational force.
Example: 100 series otter. Max take-off torque is 42.5 psi @ 97% Propeller RPM without exceeding 750 degrees T5 temp. What all that means is that the engine will produce maximum power at those figures. For a PT6A-20 that would be 550 horsepower.
It is kind of difficult to explain how torque is measured. To make it easy but vague the torque reading is the measure of Engine Oil Pressure as metered through a valve. That Oil Pressure is then picked up by an AC powered Transducer that measures that oil pressure and converts it to an electrical signal. That signal is then sent to a gauge in the cockpit where the pilot reads its value in PSI.
Now this explination is for a Twin Otter. In a King Air the system is the same, but the gauge reads in Foot Pounds. In a Caravan or PAC 750 there are no electrically powered transducers. The oil, ubder pressure, is piped right to the gauge.
On a -20 Twin Otter there are several things that would affect the proper operation of the Torque Gauge.
#1 is engine rigging. Proper engine rigging is critical on a -20 engine for everything to operate properly.
Not that it is unsafe if the engine is out of rig, but an out of rig engine will cause torque, Ng (engine speed) & prop RPMs to flucuate.
#2 is Oil level in the engine. The amount of oil will effect the "Indication" of torque. Even though the engine is producing the proper torque, the lack of sufficient oil will affect the indication..
#3 is a fault with the indicating system.
#4 is an issue with the engine or propeller.
If you would like to go into more depth than this I will be more than happy to explain further.
Hope this does answer some of your questions.

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For "not a pilot" you were pretty close. Ground effect is usually within 1/2 the wing length (1/4 total wingspan). You can feel it as the "cushion" as the airplane flares for landing (if you are still in the plane), or on a "soft field" takeoff when the pilot lifts just off the ground as early as possible and then stays in ground effect to get to normal takeoff speed.

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Ground effect doesn't increase lift, it reduces drag.

Feathering is very important for twin engined planes because the drag from a rotating prop is the same as the area of the prop arc- think of a huge plywood disc the size of the prop diameter. It isn't the drag from the blades, its from forcing the engine to turn.

Actually, forcing the engine to turn REDUCES the drag of the prop. The maximum drag is when the prop is completely freewheeling in flat pitch.

Yes and thank you. Can you define what is meant by "proper engine rigging?" Second in your #4, there is a problem with the engine or propeller, could that issue be congruent with the torque issue you mentioned in your earlier post about not enough torque to create enough rotational force to allow the props to provide the necessary horsepower? Am I at all on a track here or just completely stretching?

John, when you said "Actually, forcing the engine to turn REDUCES the drag of the prop. The maximum drag is when the prop is completely freewheeling in flat pitch." can you explain? Now I am completely confused compared to the analogy of the leading edge of the prop parallel to the ground to prevent drag. What do you mean by "freewheeling"?

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forcing the engine to turn REDUCES the drag of the prop. The maximum drag is when the prop is completely freewheeling in flat pitch."

He's talking about the props spining from wind like a windmill and not from power by the engine, the prop is not feathered and is "fat" to the relative wind, making drag.

If the prop is feathered it is skinny to the RW and makes less drag, freewheeling or not.

It's like being head down as compared to belly to earth.

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Can you define what is meant by "proper engine rigging?"

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I will try to explain "engine rigging" as it relates to adjusting throttles on the GE T58-8F engines installed in Sea King helicopters.
(Sorry, but I have only a tiny amount of experience on the Pratt & Whitney PT6A engines installed in Twin Otters).

"Rigging" throttle cables on T58 engines is a time-consuming process. It requires two or more technicians to adjust the length of throttle cables (via turn buckles) and throttle stops so that when pilots slam throttles, they get maximum rated power, but not an ounce more.

At one end of the scale: if throttle cables are too loose and stops too tight, the engine will never develop full-rated power.

At the other end of the scale, the danger is if throttle stops are so loose (or cables so short), that a pilot can slam the throttle beyond the red-line, over-heating, over-speeding or over-torquing the engine.
There are also a series of limits (bolts or valves) that limit propeller pitch, fuel flow, compressor pressure, etc. all with the goal of not over-working the engine.

Because if you over-heat or over-torque an engine, you risk damaging it, and then it goes straight to overhaul, a manyyyyy thousand dollar process.

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Correction: most torque meters are installed on output shafts ... the shaft that delivers power from the engine to the propeller.
Torque meters work on oil pressure or gear displacement.

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Because if you over-heat or over-torque an engine, you risk damaging it, and then it goes straight to overhaul, a manyyyyy thousand dollar process.

Er.....it's SUPPOSED to go straight to overhaul. A fact a couple operators will chose to ignore.

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Correction: most torque meters are installed on output shafts ... the shaft that delivers power from the engine to the propeller.
Torque meters work on oil pressure or gear displacement.

If you would like to get into detail, for a PT6 engine, that the average person will just go HUH!! at then;

The First Stage Reduction Ring Gear is allowed to "float" move fore and aft in its housing. There are grooves that aree cut on an angle on the outside of the ring that mate up with grooves in the housing.
As the Propeller starts to generate more Torque the First Stage Ring Gear then begins to move.
Riding on the forward face of the ring gear is a metering valve. As the ring gear moves aft, the metering valve is allowed to open more and more as the Torque increases. This intern ports more engine oil to the Torque Transmitter.
The Torque Transmitter is located on the Right Hand, Forward side of the Reduction Gear Box and is mount directly to a boss.
The transmitter has a vent line that runs to a fitting on the opposite side of the gear box. This line carries Case Pressure to the Transmitter.
The transmitter then uses a ratio of oil pressure/case pressure and sends an electrical signal to the indicator.
The indicator is basically a small AC powered motor. As torque increases, electrical power signal increases, motor spins faster raising the needle.
After all that it was much easier to just say what I said.

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Can you define what is meant by "proper engine rigging?"

On a -20 engine there are many linkages that all connect to one another at the front of the engine. This is called the Propeller Reversing Linkage or Beta Linkage. It is all connected to the Power Lever through a wire rope, Cam Box, Rods and cables and pulleyes. When everything is "In Rig" or properly adjusted, the engine and prop will do a certain thing at a given Power Lever setting, i.e. given torque at a given engine speed.
Now with all these linkages. Each one has a given angle, dimension or distance that it is required to be at a given power lever setting when rigging the engine. These factors need to be repeatable. Meaning once set you need to be able to move the Power Lever a couple times and reset it, this by the way is full forward, and have those angles, dimensions and distances be exactly the same as they were set.
If any of the linkages or bushings and bearings that connect them all are worn, causing slop, then the rigging will not be repeatable.
If the linkages that go to the Propeller Governor are worn or "Out of Rig" then this could cause the prop to "Hunt". This means that the prop is trying to stay at its governing speed, but can't, and the blade angles are constantly changing to compensate. This will cause a torque flucuation.
If the linkages that go to the Fuel Topping Governor are worn or "Out of Rig" this would cause ALL the engine parameters to flucuate.
Those two issues would only effect on take-off until the pilot pulls the props back for climb. Then everything tightens up and is fine.

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Second in your #4, there is a problem with the engine or propeller, could that issue be congruent with the torque issue you mentioned in your earlier post about not enough torque to create enough rotational force to allow the props to provide the necessary horsepower?

Plain and simple; Yes.

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If you would like to get into detail, for a PT6 engine, that the average person will just go HUH!! at then ...

...After all that it was much easier to just say what I said.

Thanks for the refresher. I've got lots of time working PT-6 engines, but it's been a looooong time. Nice to get a recurrency lesson occasionally.

I know crap about these engines, but I really appreciate you guys taking the time to educate me and others on them. Good info!

I know crap about them too and thanks to these fellas, I know less crap I still have questions, but I can't ask them yet. I need to let all of that digest for a while but thanks!