ASA POWERPLANT ORAL EXAM 2025
QUESTIONS AND ANSWERS
What is the main advantage of a horizontally opposed engine over a radial engine for powering
modern aircraft? - ANS The horizontally opposed engine has a much smaller frontal area and
is easier to streamline than a radial engine
How many throws are there in the crankshaft of a six-cylinder horizontally opposed engine? -
ANS Six
What kind of connecting rod arrangement is used in a radial engine? - ANS A master rod
connects the single throw of the crankshaft with a piston. All of the other pistons are connected
to the master rod with link rods
Of what material are most piston rings made? - ANS Gray cast iron
What is the reason for using hydraulic valve lifters in an aircraft engine? - ANS Hydraulic
valve lifters keep all of the clearance out of the valve operating mechanism. This decreases the
wear of the valve train components
At what speed does the camshaft turn, relative to the crankshaft speed in a horizontally
opposed engine? - ANS The camshaft turns at one half of the crankshaft speed
What kind of main bearings are used in a horizontally opposed engine? - ANS Steel-backed,
lead-alloy bearing inserts
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, On what stroke is the piston of a reciprocating engine when the intake valve begins to open? -
ANS On the exhaust stroke
On what stroke is the piston of a reciprocating engine when the exhaust valve begins to open? -
ANS On the power stroke
Why are both the hot and cold valve clearances give for most radial engine? - ANS The hot
clearance is given for valve timing purposes. The timing is adjusted with the valves in cylinder
number one, set with the hot clearance. When the timing is set, all of the valves are adjusted to
their cold clearance.
What is meant by a cam-ground piston? - ANS A piston that is not perfectly round. Its
dimension parallel with the wrist pin is several thousandths of an inch less than its dimension
perpendicular to the wrist pin. When the piston reaches operating temperature, the metal in
the piston pin boss expands enough that the piston becomes perfectly round
Where is the piston in a reciprocating engine when the ignition spark occurs? - ANS About 30
degrees of crankshaft rotation before the piston reaches top center on the compression stroke
What is meant by a full-floating wrist pin? - ANS A wrist pin that is not clamped in either the
piston or the connecting rod. Full-floating wrist pins are kept from scoring the cylinder walls by
soft metal plugs in their ends
Why do most aircraft reciprocating engines use more than one spring on each valve? -
ANS By using more than one spring and having the wire diameter and pitch of the springs
different, valve float is minimized. The springs have different resonant frequencies, so at least
one spring will always be exerting a force on the valve
Would excessive valve clearance cause the valves to open early or late? - ANS Excessive
clearance will cause the valve to open late and close early
What is the purpose of valve overlap in a reciprocating engine? - ANS Valve overlap allows
2
the inertia of the exhaust gases leaving the cylinder to help the fresh induction charge start
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flowing into the cylinder
@COPYRIGHT FYNDLAY 2025/2026
, What type of piston rings are installed on the pistons of an aircraft reciprocating engine? -
ANS Compression rings, oil control rings, and oil wiper rings
Why are some exhaust valves partially filled with metallic sodium? - ANS The metallic sodium
melts at engine operating temperature and sloshes back and forth inside the hollow valve. It
picks up heat from the valve head and transfers it into the valve stem, so it can be transferred
to the cylinder head through the valve guide
What causes detonation in an aircraft engine? - ANS Excessive heat and pressure in the
engine cylinder causes the fuel-air mixture to reach its critical pressure and temperature. Under
these conditions, the mixture explodes rather than burns. This explosion is called detonation
Why is a compression check important for determining the condition of an aircraft
reciprocating engine? - ANS A compression check can determine the condition of the seal
between the piston rings and the cylinder walls, and the seal between the intake and exhaust
valves and their seats
What is meant by the compression ratio of a reciprocating engine? - ANS The ratio of the
volume of the cylinder with the piston at the bottom of its stroke to the volume with the piston
at the top of its stroke
What are the two basic sections of a turbine engine? - ANS The hot section and the cold
section
What are the two basic types of compressors that are used in aircraft turbine engines? -
ANS Axial-flow and centrifugal
What is the purpose of the stators in an axial-flow compressor? - ANS The stators convert
some velocity energy into pressure energy and change the direction of the air so it is proper for
the next stage of rotors
3
Page
@COPYRIGHT FYNDLAY 2025/2026
QUESTIONS AND ANSWERS
What is the main advantage of a horizontally opposed engine over a radial engine for powering
modern aircraft? - ANS The horizontally opposed engine has a much smaller frontal area and
is easier to streamline than a radial engine
How many throws are there in the crankshaft of a six-cylinder horizontally opposed engine? -
ANS Six
What kind of connecting rod arrangement is used in a radial engine? - ANS A master rod
connects the single throw of the crankshaft with a piston. All of the other pistons are connected
to the master rod with link rods
Of what material are most piston rings made? - ANS Gray cast iron
What is the reason for using hydraulic valve lifters in an aircraft engine? - ANS Hydraulic
valve lifters keep all of the clearance out of the valve operating mechanism. This decreases the
wear of the valve train components
At what speed does the camshaft turn, relative to the crankshaft speed in a horizontally
opposed engine? - ANS The camshaft turns at one half of the crankshaft speed
What kind of main bearings are used in a horizontally opposed engine? - ANS Steel-backed,
lead-alloy bearing inserts
1Page
@COPYRIGHT FYNDLAY 2025/2026
, On what stroke is the piston of a reciprocating engine when the intake valve begins to open? -
ANS On the exhaust stroke
On what stroke is the piston of a reciprocating engine when the exhaust valve begins to open? -
ANS On the power stroke
Why are both the hot and cold valve clearances give for most radial engine? - ANS The hot
clearance is given for valve timing purposes. The timing is adjusted with the valves in cylinder
number one, set with the hot clearance. When the timing is set, all of the valves are adjusted to
their cold clearance.
What is meant by a cam-ground piston? - ANS A piston that is not perfectly round. Its
dimension parallel with the wrist pin is several thousandths of an inch less than its dimension
perpendicular to the wrist pin. When the piston reaches operating temperature, the metal in
the piston pin boss expands enough that the piston becomes perfectly round
Where is the piston in a reciprocating engine when the ignition spark occurs? - ANS About 30
degrees of crankshaft rotation before the piston reaches top center on the compression stroke
What is meant by a full-floating wrist pin? - ANS A wrist pin that is not clamped in either the
piston or the connecting rod. Full-floating wrist pins are kept from scoring the cylinder walls by
soft metal plugs in their ends
Why do most aircraft reciprocating engines use more than one spring on each valve? -
ANS By using more than one spring and having the wire diameter and pitch of the springs
different, valve float is minimized. The springs have different resonant frequencies, so at least
one spring will always be exerting a force on the valve
Would excessive valve clearance cause the valves to open early or late? - ANS Excessive
clearance will cause the valve to open late and close early
What is the purpose of valve overlap in a reciprocating engine? - ANS Valve overlap allows
2
the inertia of the exhaust gases leaving the cylinder to help the fresh induction charge start
Page
flowing into the cylinder
@COPYRIGHT FYNDLAY 2025/2026
, What type of piston rings are installed on the pistons of an aircraft reciprocating engine? -
ANS Compression rings, oil control rings, and oil wiper rings
Why are some exhaust valves partially filled with metallic sodium? - ANS The metallic sodium
melts at engine operating temperature and sloshes back and forth inside the hollow valve. It
picks up heat from the valve head and transfers it into the valve stem, so it can be transferred
to the cylinder head through the valve guide
What causes detonation in an aircraft engine? - ANS Excessive heat and pressure in the
engine cylinder causes the fuel-air mixture to reach its critical pressure and temperature. Under
these conditions, the mixture explodes rather than burns. This explosion is called detonation
Why is a compression check important for determining the condition of an aircraft
reciprocating engine? - ANS A compression check can determine the condition of the seal
between the piston rings and the cylinder walls, and the seal between the intake and exhaust
valves and their seats
What is meant by the compression ratio of a reciprocating engine? - ANS The ratio of the
volume of the cylinder with the piston at the bottom of its stroke to the volume with the piston
at the top of its stroke
What are the two basic sections of a turbine engine? - ANS The hot section and the cold
section
What are the two basic types of compressors that are used in aircraft turbine engines? -
ANS Axial-flow and centrifugal
What is the purpose of the stators in an axial-flow compressor? - ANS The stators convert
some velocity energy into pressure energy and change the direction of the air so it is proper for
the next stage of rotors
3
Page
@COPYRIGHT FYNDLAY 2025/2026
