Airplane Systems questions.

[Keith.Williams.]

Q102)
When leaning the mixture for the most economic cruise
fuel flow, excessive leaning will cause :
A) high engine rpm
B)low cylinder head and exhaust gas temperature (what I think)
C) high manifold pressure
D) high cylinder head and exhaust gas temperature (according to the mark scheme)

The maximum CHT occurs at 1/15 fuel/air ratio. For most engines the economy setting is just rich of this ratio. So if you lean the mixture excessively you will be closer to the 1/15 ratio. This will give a higher temperature. giving a higher temperature. So the answer is D.

Q525)
The speed of an asynchronous four-pole motor fed at a
frequency of 400 Hertz is:
A) 800 revolutions per minute.
B) 1600 revolutions per minute.
C) 12000 revolutions per minute. (according to the mark scheme)
D) 6000 revolutions per minute. (what I think)

The formula you were thinking of is Frequency = (RPM x Pole Pairs) / 60 or if you prefer Frequency = (RPM x Poles ) / 120. This gives 120oo RPM. The question is however a little bit defective in that is specifies an "asynchronous motor". These are induction motors which exhibit a slip speed, which (depending on load applied) is typically about 10% of synchronous RPM. So we should really be looking for 12000 - 10%, but this isn't an option. So the bast option is C.

Q571)
If an engine fails to stop with the magneto switch in OFF
position, the cause may be :
A) defective condenser
B) fouled spark plugs
C) excessive carbon formation in cylinder head. (according to the mark scheme)
D) switch wire grounded (what I think)

Piston engines will stop when the ignition is switched off unless some other ignition source is available. One such source is hot carbon deposites within the cylinder. Magnetos are switched off by using the ignition switch to ground (or earth) the primary coil. So if the ignition switch is grounded the magneto will be permanently switched off. So option D is incorrect and option C is the best one in this question. But a better option would be "igintion switch disconnecetd" if it were available, bcause this would make it impossible to switch off the magneto.

Q583)
The conditions under which you obtain the highest
engine power are :
A) cold and humid air at high pressure. (what I think)
B) warm and dry air at high pressure.
C) cold and dry air at high pressure. (according to the mark scheme)
D) warm and humid air at low pressure.

The power output of any engine is proportional to the mass flow of air passing through it. Air mass flow rate is proportional to air density. At any given RPM the mass flow and power output will be greatest when operating in high pressure cold air because this gives maximum air density and hence maximum mass flow rate.

Q393)
On a a normally aspirated engine (non turbo-charged),
the manifold pressure gauge always indicates...
A) a value equal to the QFE when the engine is at full power on
the ground. (what I think)
B) a lower value than atmospheric pressure when the engine is
running. (according to the mark scheme)
C) a greater value than atmospheric pressure when the engine
is running.
D) zero on the ground when the engine is stopped.

Normally aspirated engines must suck the air in through the throttle valve and choke tube into the inlet manifold. MAP is the air pressure within the inlet manifold. The restriction caused by the throttle valve and choke tube causes the pressure to drop. So whenever a normally aspirated engien is running the mAP will be les than the ambient pressure.

Q389)
During climb with constant Manifold Air Pressure (MAP)
and RPM indication and constant mixture setting, the
power output of a piston engine :
A) stays constant.
B) increases. (according to the mark scheme)
C) decreases. (what I think)
D) only stays constant if the speed control lever is pushed
forward.

In order to maintain constant MAP in a climb, you must start with the throttle valve only partly open and gradually open it to compensate for the reducing ambient pressure as you climb. If you do this such that MAP is constant, you will have a constant pressure pushing air from the manifold into the cylinders.

But as altitude increases, the decreasing ambient pressure will make it easier for the exhaust gas to escape through the exhaust pipe. (Reducing exhaust back pressure). This constant inlet pressure and reducing exhaust back pressure will make it easier for mixture to flow into the cylinder. So the mass flow of mixture through the engine, and hence the power output will increase.




Q320)
The advantages of a chemical oxygen source for the
passenger cabin are :
1. reduced weight and volume,
2. easy storage and maintenance,
3. greater autonomy,
4. no risk of explosion,
5. reversible functioning,
6. no maintenance.
The combination regrouping all the correct statements
is:
A) 1, 2, 4, 6 (according to the mark scheme)
B) 1, 2, 3, 4, 5, 6
C) 2, 3, 5
D) 1, 3, 4, 5

Oxygen generators are simply light aluminium tube sfilled with solid chemicals. So they are much lighter than the high-pressure cylinders that are used in gaseous oxygen systems. So 1 is correct.

The chemical generators are fitted to the aircarft then require no maintenance for many years. But hey must be replaced every 5 years or so. So option 2 is correct(ish).

The word "autonomy" means the ability to make decisions without the need for the consent of others. So statement 3 is a bit dubious. But if it is taken to mean "the facility for automatic operation, such as variation of flow rate and oxygen/air ratio", then it is not true of a chemical oxygen generator. Having been activated they simply produce a trickle flow of oxygen until the chemicals have been exhausted. So statement 3 is incorrect.

Strictly speaking no oxygen system is totally imune to risk of explaosion. A recent accident in a submarine involved an explaosion in a chemical oxygen generator. But chemical generator are far less likley to explode than are high pressure gas cylinedrs. So stement 4 is true(ish)

The operation of chemical generator is not reversible in that they cannot be recharged in the sma eway a high pressure cylinders can. In reality a used chemical generator must be replaced. So statement 5 is untrue.

Chemical generators require no maintenance until they are replaced after 5 years or so, when their life expires. So statment 6 is true(ish).

Taking all of the above into accout, option 1 is the most accurate in this question.


The power output of a normally aspirated piston engine
increases with increasing altitude at constant Manifold
Air Pressure (MAP) and RPM because of the :
A) lower losses during the gas change.
B) lower friction losses.
C) leaner mixture at higher altitudes.
D) lower back pressure. (mark scheme answer)

In order to maintain constant MAP in a climb, you must start with the throttle valve only partly open and gradually open it to compensate for the reducing ambient pressure as you climb. If you do this such that MAP is constant, you will have a constant pressure pushing air from the manifold into the cylinders.

But as altitude increases, the decreasing ambient pressure will make it easier for the exhaust gas to escape through the exhaust pipe. (Reducing exhaust back pressure). This constant inlet pressure and reducing exhaust back pressure will make it easier for mixture to flow into the cylinder. So the mass flow of mixture through the engine, and hence the power output will increase.
So option D is correct.



Q14)
Internal leakage in a hydraulic system will cause :
A) an increased fluid pressure.
B) an increased fluid temperature. (mark scheme answer)
C) fluid loss.
D) a decreased fluid temperature.

Internal leaks will not cause loss of fluid. BUt they will cause system pressure to decrease.

In modern systems the pumps go to zero output, but system pressure remains constant when the services are not being used

But to maintain system pressure when there is an internal leak, the pumps must continue to push fluid through the system. This will cause the fluid temperature to increase.

The main problem with most (but not all) of the question banks currently available is the fact that they contain very few (if any) meaningful explanations. This means that users are simply learning the answers to questions.