AMT-P Powerplant Oral Questions and
Answers
What is the main advantage of a horizontally opposed engine over a radial engine for powering
modern aircraft - answerThe horizontally opposed engine has a much smaller frontal area & is
easier to streamline than a radial engine.
How many throws are there in the crankshaft of a six-cylinder horizontally opposed engine -
answerSix.
What kind of connecting rod arrangement is used in a radial engine - answerA master rod
connects the single throw of the crankshaft with the piston. All of the other pistons are connected
to the master rod with link rods.
Of what material are most piston rings made - answerGray cast iron.
What is the reason for using hydraulic valve lifters in an aircraft engine - answerHydraulic 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 - answerThe camshaft turns at one half of the crankshaft speed.
What kind of main bearings are used in a horizontally opposed engine - answerSteel-backed,
lead-alloy bearing inserts.
On what stroke is the piston of a reciprocating engine when the intake valve begins to open -
answerOn the exhaust stroke.
,On what stroke is the piston of a reciprocating engine when the exhaust valve begins to open -
answerOn the power stroke.
Why are both the hot & cold valve clearances given for most radial engines - answerThe 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 - answerA 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 - answerAbout 30
degrees of crankshaft rotation before the piston reaches top center of the compression stroke.
What is meant by a full-floating wrist pin - answerA 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 - answerBy
using more than one spring & having the wire diameter & pitch of the spring 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 - answerExcessive valve
clearance will cause the valve to open late & close early.
What is the purpose of valve overlap in a reciprocating engine - answerValve overlap allows the
inertia of the exhaust gases leaving the cylinder to help the fresh induction charge start flowing
into the cylinder.
,What type of piston rings are installed on the pistons of an aircraft reciprocating engine -
answerCompression rings, oil control rings, & oil wiper rings.
Why are some exhaust valves partially filled with metallic sodium - answerThe metallic sodium
melts at engine operating temperature & sloshes back & forth inside the hollow valve. It picks up
heat from the valve head & 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 - answerExcessive heat & pressure in the engine
cylinder causes the fuel-air mixture to reach its critical pressure & 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 - answerA compression check can determine the condition of the seal between the piston
rings & the cylinder walls, & the seal between the intake & exhaust valves & their seats.
What is meant by the compression ratio of a reciprocating engine - answerThe 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.
In what position should the crankshaft be when a magneto is being timed to the engine -
answerThe piston in cylinder no. 1 should be in the correct position for ignition to occur. This is
normally about 30° before top center on the compression stroke.
What is the purpose of a cold cylinder check - answerA cold cylinder check determines which
cylinder is not firing. Run the engine for a few minutes at the speed, and on the magneto, at which
it runs roughest. Shut it down and feel the exhaust stack near the cylinder head. The cylinder with
the stack that is not as hot as the others is the cylinder that has not been firing.
What is checked on engine runup to determine the operational condition of the engine -
answerIdling RPM and manifold pressure, engine acceleration, maximum static RPM and
manifold pressure, magneto drop, ignition switch safety check, propeller pitch change, propeller
feathering, oil pressure, fuel pressure, & fuel flow.
, How can you determine the firing order of an 18-cylinder radial engine - answerBegin with
cylinder 1, add 11, and subtract 7, whichever will keep the numbers between 1 and 18. 1-12-5-
16-9-2-13-6-17-10-3-14-7-18-11-4-15-8.
What are two main causes of a cylinder having low compression - answerPiston rings not seating
or broken, and valves not seating or burned.
What is a hydraulic lock in an aircraft engine - answerOil has drained past the piston rings and
filled the combustion chamber of cylinders that are below the center line of the engine. If the
crankshaft is turned over with this oil in the cylinder it will lock, and there is a probability that the
piston, cylinder, or connecting rod will be damaged.
How is a hydraulic lock removed from an aircraft engine - answerRemove a spark plug from the
locked cylinder and drain all of the oil out. Clean the spark plug and replace it.
What is a low-tension ignition system - answerIt is an ignition system using a magneto with a
brush-type distributor and a coil that has no secondary wiring. Low voltage is directed from the
magneto to a transformer mounted on the cylinder head. Here the low voltage is boosted to a high
voltage and is directed through a very short high-tension lead to a spark plug. There is one
transformer for each spark plug.
What is a floating cam ring on a large radial engine - answerIt is a cam ring that rides on a shelf-
type bearing with a large amount of clearance between the bearing and the ring. When adjusting
the valves on an aircraft engine with a floating cam ring, the pressure of the valve springs on the
opposite side of the engine must be removed so the cam ring will be tight against the bearing for
the valves being adjusted.
Why is valve clearance adjusted on radial engines, but not on most horizontally opposed engines -
answerMost horizontally opposed engines have hydraulic valve lifters that keep all of the clearance
out of the valve train. Radial engines have solid lifters.
What must be inspected to determine the airworthiness of an engine installation - answerThe
Answers
What is the main advantage of a horizontally opposed engine over a radial engine for powering
modern aircraft - answerThe horizontally opposed engine has a much smaller frontal area & is
easier to streamline than a radial engine.
How many throws are there in the crankshaft of a six-cylinder horizontally opposed engine -
answerSix.
What kind of connecting rod arrangement is used in a radial engine - answerA master rod
connects the single throw of the crankshaft with the piston. All of the other pistons are connected
to the master rod with link rods.
Of what material are most piston rings made - answerGray cast iron.
What is the reason for using hydraulic valve lifters in an aircraft engine - answerHydraulic 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 - answerThe camshaft turns at one half of the crankshaft speed.
What kind of main bearings are used in a horizontally opposed engine - answerSteel-backed,
lead-alloy bearing inserts.
On what stroke is the piston of a reciprocating engine when the intake valve begins to open -
answerOn the exhaust stroke.
,On what stroke is the piston of a reciprocating engine when the exhaust valve begins to open -
answerOn the power stroke.
Why are both the hot & cold valve clearances given for most radial engines - answerThe 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 - answerA 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 - answerAbout 30
degrees of crankshaft rotation before the piston reaches top center of the compression stroke.
What is meant by a full-floating wrist pin - answerA 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 - answerBy
using more than one spring & having the wire diameter & pitch of the spring 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 - answerExcessive valve
clearance will cause the valve to open late & close early.
What is the purpose of valve overlap in a reciprocating engine - answerValve overlap allows the
inertia of the exhaust gases leaving the cylinder to help the fresh induction charge start flowing
into the cylinder.
,What type of piston rings are installed on the pistons of an aircraft reciprocating engine -
answerCompression rings, oil control rings, & oil wiper rings.
Why are some exhaust valves partially filled with metallic sodium - answerThe metallic sodium
melts at engine operating temperature & sloshes back & forth inside the hollow valve. It picks up
heat from the valve head & 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 - answerExcessive heat & pressure in the engine
cylinder causes the fuel-air mixture to reach its critical pressure & 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 - answerA compression check can determine the condition of the seal between the piston
rings & the cylinder walls, & the seal between the intake & exhaust valves & their seats.
What is meant by the compression ratio of a reciprocating engine - answerThe 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.
In what position should the crankshaft be when a magneto is being timed to the engine -
answerThe piston in cylinder no. 1 should be in the correct position for ignition to occur. This is
normally about 30° before top center on the compression stroke.
What is the purpose of a cold cylinder check - answerA cold cylinder check determines which
cylinder is not firing. Run the engine for a few minutes at the speed, and on the magneto, at which
it runs roughest. Shut it down and feel the exhaust stack near the cylinder head. The cylinder with
the stack that is not as hot as the others is the cylinder that has not been firing.
What is checked on engine runup to determine the operational condition of the engine -
answerIdling RPM and manifold pressure, engine acceleration, maximum static RPM and
manifold pressure, magneto drop, ignition switch safety check, propeller pitch change, propeller
feathering, oil pressure, fuel pressure, & fuel flow.
, How can you determine the firing order of an 18-cylinder radial engine - answerBegin with
cylinder 1, add 11, and subtract 7, whichever will keep the numbers between 1 and 18. 1-12-5-
16-9-2-13-6-17-10-3-14-7-18-11-4-15-8.
What are two main causes of a cylinder having low compression - answerPiston rings not seating
or broken, and valves not seating or burned.
What is a hydraulic lock in an aircraft engine - answerOil has drained past the piston rings and
filled the combustion chamber of cylinders that are below the center line of the engine. If the
crankshaft is turned over with this oil in the cylinder it will lock, and there is a probability that the
piston, cylinder, or connecting rod will be damaged.
How is a hydraulic lock removed from an aircraft engine - answerRemove a spark plug from the
locked cylinder and drain all of the oil out. Clean the spark plug and replace it.
What is a low-tension ignition system - answerIt is an ignition system using a magneto with a
brush-type distributor and a coil that has no secondary wiring. Low voltage is directed from the
magneto to a transformer mounted on the cylinder head. Here the low voltage is boosted to a high
voltage and is directed through a very short high-tension lead to a spark plug. There is one
transformer for each spark plug.
What is a floating cam ring on a large radial engine - answerIt is a cam ring that rides on a shelf-
type bearing with a large amount of clearance between the bearing and the ring. When adjusting
the valves on an aircraft engine with a floating cam ring, the pressure of the valve springs on the
opposite side of the engine must be removed so the cam ring will be tight against the bearing for
the valves being adjusted.
Why is valve clearance adjusted on radial engines, but not on most horizontally opposed engines -
answerMost horizontally opposed engines have hydraulic valve lifters that keep all of the clearance
out of the valve train. Radial engines have solid lifters.
What must be inspected to determine the airworthiness of an engine installation - answerThe
