4) Seals

Apparently the oil acts as a seal. This was a favorite question the examiner used to throw at us during the FAA orals, and luckily I had the same examiner more than once ;)

Ahh, rings a bell...I think. What does it help seal? Gas from the crankcase at the piston rings perchance?

I think the other major things are:

* The prop CSU
* Turbo supercharger waste gate actuation.

And is the gauge likely to tell you anything useful if you have an engine failure? Many would expect it to show a reduced MP.

M.

? Sealing

Flyboy F33 - Many thanks for your response. :ok:

That's an easy one. If the engine is normally aspirated the MP will be below ambient pressure when the engine is running (or even being cranked by a windmilling prop). It will obviously revert to ambient once the engine has stopped. If the engine is turbo supercharged then it will drop if the MP was above ambient (toga or climb power usually).

But what does MP indicate?

Oil is also used for:-

I'm intrigued by this, for in all the applications I know of (automotive admittedly), the wastegate valve is operated either by vacuum or electronically. Can anyone expand a little on this please?

Why do most fuel systems have a restricted return line from near the engine back into the fuel tank.

(Not a simple question or answer that, but will certainly concentrate many minds on how their aeroplanes work).

G

Kolibear,

Something like this, but I can't speak for all systems, this is the scheme examined by the CAA:

Basically, the wastegate is controlled directly by oil pressure. High pressure closes the wastegate. In the inlet manifold, there is an aneroid capsule with a needle valve that controls the flow of the oil. When manifold pressure reduces, the capsule expands and close the needle value, stoping the flow of oil and so closing the wastegate so that more or all gas is directed to the turbine. When the manifold pressure increases, the capsule contracts openning the needle valve, letting the oil flow and so openning the waste gate partially or fully.

Keygrip
 

If the problem with the PA44 gear was just a bulb the gear in transit light ( gear switch/gear position disagree light ) would NOT be on.
If the gear was Not in the position selected then the transit light WOULD be on.

In effect the system has two gear warning systems.

A and C, correct. Clever system really.

GtE
If I remember something I read once correctly, in a system whereby the fuel is pumped at high pressure, ie an electric pump as opposed to an engine driven pump, there is a danger, albeit it remote, that this pressure could result in the float chamber valve not being able to seat correctly, producing flooding and the associated risk of fire.
Don't know if this is anything to do with it, just an educated guess based on information from back of mind. At least if this question were to be asked of me, I think this is the answer I would come up with, rather than a 'blank stare'.

TJ

Well i'll throw my hat into the ring...

How about the "absolute pressure of the fuel-air charge at the intake manifold"?

Whether it's fuel/air or only air will depend on the fuel delivery sytstem and the exact position of the 'sensor'.

Did I pass or do I have to re-sit Chimbu? ;)

Told you it was a good question, but that wouldn't be my answer. I'll let it sit for a few days I think :cool:

G

Hmmm....the absolute pressure of the gas at the MAP gauge calibrated for a particular temperature.

Chimbu,

The MP provides an indication of engine power.

Genghis,

Isn't the fuel return line on relevant for fuel injection systems?

hot start on fuel injected systems, could be vapour lock, so turning the pump on allows cold fuel from engine to fill the pipes, and the hot gassy stuff is returned to the tank via the return line ???????

Nearly a textbook answer SS. Vapour locking can cause a large vapour bubble in the fuel line which maintains a small amount of fuel in the carb bowl / injector, normal fuel pressure - but means that you've got stored up a period when suddenly fuel stops flowing (usually just as you have left the ground!).

A return line means a continuous trickle of cold fuel through the lines, and also an escape route for vapour bubbles. Thus, there's a much reduced risk of vapour locking - which otherwise can cause an EFATO (with no subsequent fault found usually) after the aircraft has been sat in the sun for a while (sun! - remember that?).

G

All other things being constant, i.e. a CS prop, MP is a cheap way of measuring torque.

Unfortunately, since the inlet valves insulate the breathing side of the engine from the combustion side of the engine, a given engine going around at a given RPM, with a given throttle opening, will read the same MP (even if no combustion is actually taking place)...

The nearest to a direct measure of engine power is the fuel flow, assuming one is leaned at or past peak EGT.