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Confusion with question
Sneh created the topic: Confusion with question
Exercise GK 2 Question 1:
The manifold pressure gauge on an inoperative engine would be expected to read:
C) the ambient atmospheric pressure
based on the information given on page 2.7 of CPL General Knowledge which states the "When the aircraft is parked with engine inoperative, the normal outside pressure floods through the inlet manifold, so that at sea-level the manifold pressure gauge would read about 30HG"
Based on this information i selected 'D) 30Hg"' however i got it wrong because the correct answer is 'C) the ambient atmospheric pressure'. Can some please explain me why is the correct answer? I have gone through the explanation which is present next to the answer but got confused. Any form of explanation will be much appreciated.
John.Heddles replied the topic: Confusion with question
The manifold pressure gauge is just a manometer. This is a gadget which measures and tells you the local air pressure if the engine is not running. It does no more, no less.
The local air pressure varies according to where you are in the atmosphere and gives you a reading dependent on how much air is above your location. The air pressure is a measure of the weight of the air above you.
If you are lower in the atmosphere, say near sea level, there is more air above you so it weighs more at your location and the pressure is higher. The manifold pressure gauge shows a bigger (higher) pressure reading.
Alternatively, if you are higher in the atmosphere, say at the top of Mt Kosciuszko, there is less air above you so it weighs less at your location and the pressure is lower. The manifold pressure gauge shows a smaller (lower) pressure reading.
Now, for pilots, the only pressure value (as a number) we keep in mind is sea level pressure, which (under ISA conditions) is around 1013 mb (or hPa, as we say these days) or, if you want to use the US system, 29.92 inHg. You can use other units if you wish but these two are the most common that you will see. So, if you just happen to be somewhere where the air pressure is the same as ISA sea level, then your manifold pressure gauge will read one of these figures depending in its calibration. If you are higher in the atmosphere, the gauge will read a lower pressure, if you are lower, it will read a higher pressure. The important thing to remember is that it reads the local pressure, whatever that may be.
As most of our manifold pressure gauges originate in the US, we commonly see the scale calibrated in inHg (inches of mercury). This measure is the height of mercury in a glass column where the height balances the weight of the mercury against the weight (pressure) of the air.
Strictly, this applies only when the engine is not running. In that case, the gauge is sensing the local atmospheric pressure. If the engine is running, the gauge, sensing the pressure in the intake manifold, will be seeing a very much reduced pressure due to the way the intake manifold operates. The reading will vary with throttle setting, ranging from a very low reading with the throttle closed up to a reading a bit less than the local atmospheric pressure with full open throttle.
Looking at the question, we can apply the previous discussion to each answer in turn
(a) the MP will read 1013.2 ONLY IF we are at ISA sea level pressure conditions. Generally, this will not be the case.
(b) if the MP reads zero either it is broken or we are in a vacuum (ie outer space). Maybe not a feasible answer.
(c) the MP will always read the local pressure in the intake manifold. If the engine is running, this is the intake pressure. If the engine is not running, this is the ambient atmospheric pressure. Definitely a good answer.
(d) the MP will read 30inHg if we happen to be at an elevation where that is the ambient pressure. This elevation would be a little below ISA sea level. Not generally the case.
So (c) is the best answer as it is the most generally correct. (a) and (d) may be correct but they are not generally going to be the case so we can reject them as being inferior to (c). (b) we can reject as a distractor.
The question is a bit searching in that it needs the student to have a sound idea of what the MP gauge is actually doing.
Engineering specialist in aircraft performance and weight control.