DGCA Technical General: Fuel, Mixture & Carburation Questions
78 exam-style Fuel, Mixture & Carburation questions for the DGCA Technical General paper (CPL · ATPL · PPL). The first 12 are free below with answers and explanations — sign up free to practise all of them with progress tracking.
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Q1
If the specific gravity of a fuel is known to be 0.7, 100 Imperial Gallons of it will weigh:
- A 700 Ib
- B 70 Ib
- C 7000 Ib
- D 7,100 Ib
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Correct answer: A. 700 Ib
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Q2
In a modern carburettor, mixture is controlled via
- A Airflow
- B Airflow, fuel flow and temperature
- C Fuel flow
- D Airflow and fuel flow
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Correct answer: C. Fuel flow
In a modern carburettor the pilot's mixture control adjusts the quantity of fuel metered into the fixed induction airflow, so mixture is set by controlling fuel flow. Leaning at altitude simply reduces the fuel delivered for the air being drawn in.
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Q3
246.When an engine is fitted with a fuel injection system:
- A It does not require priming.
- B Priming fuel originates from the excess supplied from the engine driven pump
- C A separate priming system is not requirede
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Correct answer: C. A separate priming system is not requirede
A fuel injection system delivers metered fuel directly to the induction ports or cylinders, so operating the boost pump with the mixture set rich charges the lines and primes the engine through the injectors themselves. No separate priming system is therefore required, although the engine still needs priming fuel for a cold start.
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Q4
The method of priming an engine not fitted with a priming pump is to:
- A Pump the throttle several times.
- B Turn the engine over several times on the starter-motor before selecting the ignition on.
- C Position the throttle lever midway between open and close.
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Correct answer: A. Pump the throttle several times.
Pumping the throttle several times operates the carburettor's accelerator pump, which squirts raw fuel into the induction system and enriches the mixture for starting. This substitutes for a priming pump on engines not fitted with one, though it carries an increased risk of fire in the induction system if overdone.
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Q5
Weakening the mixture below the best fuel/air ratio will cause the engine power to:
- A Decrease.
- B Increase initially, but decrease below take off power.
- C Increase.
- D Be unaffected by altitude increase.
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Correct answer: A. Decrease.
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Q6
What does ‘octane rating’ when applied to AVGAS refer to:-
- A The waxing point of the fuel
- B The ability of the fuel to disperse water
- C The anti-knock value of the fuel
- D The volatility of the fuel
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Correct answer: C. The anti-knock value of the fuel
For AVGAS the octane (performance) rating expresses the fuel's anti-knock quality, that is its resistance to detonation under compression and heat. A higher rating means the fuel can withstand higher cylinder pressures without knocking.
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Q7
The pressure in the induction manifold of a normally aspirated engine:
- A Remains constant as the throttle is opened.
- B Decreases as the throttle is opened.
- C Initially increases as the throttle is opened but decreases after approximately the half open position.
- D Increases as the throttle is opened.
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Correct answer: D. Increases as the throttle is opened.
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Q8
The engine driven fuel pump supplies:
- A The exact amount of fuel required for all running conditions.
- B More fuel than is required by the engine; the excess fuel is recycled.
- C The exact amount of fuel required for all running and starting conditions.
- D More fuel than is required by the engine, the excess being used as priming fuel.
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Correct answer: B. More fuel than is required by the engine; the excess fuel is recycled.
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Q9
The EGT indication on a piston engine is used
- A To control the cooling air shutters
- B To monitor the oil temperature
- C To assist the pilot to adjust the fuel mixture
- D To indicate cylinder head temperature
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Correct answer: C. To assist the pilot to adjust the fuel mixture
On a piston engine the exhaust gas temperature indication is used to help the pilot lean the mixture, since peak EGT identifies the chemically correct fuel-air ratio from which the mixture can be set for best power or best economy.
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Q10
A fuel grade which is used in typical aircraft engines is:
- A D.T.D. 585/100
- B D.E.R.D. 2479
- C AVGAS 100
- D D.E.R.D. 2484
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Correct answer: C. AVGAS 100
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Q11
In a normally aspirated piston engine carburettor icing can occur:
- A Between 0oC and –10oC
- B At more than + 10oC
- C Only at less than + 10oC if there is visible moisture
- D Only above 5000 ft
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Correct answer: C. Only at less than + 10oC if there is visible moisture
Carburettor icing forms when the pressure drop and fuel vaporisation in the venturi cool the induction air below freezing, so it can occur in ambient conditions at or below about +10°C provided there is visible moisture or high humidity. It is not confined to sub-zero air or to high altitude.
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Q12
For maximum endurance the mixture control should be set to:
- A Weak.
- B The chemically correct state.
- C Between rich and weak.
- D Rich.
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Correct answer: A. Weak.
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