Engine Instrument Systems — FAA A&P Test Questions (ACS AM.III.D)

What ACS AM.III.D covers

This area covers the sensing, transmitting, and indicating hardware that reports engine condition to the flight deck on both reciprocating and turbine engines, and the descriptions the FAA tests against are the ones in the Powerplant Handbook, FAA-H-8083-32. Temperature indication is the biggest single block. Cylinder head temperature and turbine exhaust gas temperature are both thermocouple systems: two dissimilar metals joined at a hot junction produce a small voltage proportional to the temperature difference between that junction and the cold, or reference, junction at the indicator. Because the indicator is calibrated for a fixed external circuit resistance, replacement thermocouple leads must match the specified resistance, which is why leads come as matched sets and are not shortened or spliced at will. Pressure indication is the second block: the Bourdon tube is a curved, flattened tube that tries to straighten as internal pressure rises, and that motion is geared to the pointer on oil pressure and fuel pressure gauges. The manifold absolute pressure gauge measures absolute pressure in the intake manifold, so on a shut-down engine at sea level it reads close to local barometric pressure, not zero, and in flight it is your reciprocating power indication alongside rpm. Speed indication covers mechanical tachometers, the electric tachometer system in which a small engine-driven AC generator drives a synchronous motor in the indicator, and turbine tachometers, which display percent of rated rpm rather than actual rpm. Turbine questions also use station notation, where a subscript t means total pressure and the number identifies the engine station the sensor reads, as in Pt7 at the exhaust end of the engine.

Where this sits on the test

ACS AM.III.D is tested on the FAA Powerplant written test, one of 1,355 ACS-tagged questions in the Powerplant bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

• FAA-H-8083-32

Three traps candidates fall into

1. Candidates expect a manifold pressure gauge to read zero with the engine stopped. It is an absolute gauge, so with the engine shut down at sea level it reads close to local barometric pressure, roughly 30 inches of mercury. A reading well below that on a dead engine suggests a gauge or line problem.
2. Thermocouple leads get treated as ordinary wire. The indicator is calibrated against a specified total lead resistance, so trimming, lengthening, or substituting leads changes the resistance and shifts every temperature reading. Replace them as the matched set the manufacturer specifies rather than fabricating a lead from spare wire.
3. Turbine tachometers get read as revolutions per minute. They indicate percent of the manufacturer's rated rpm, so 100 percent is a reference speed rather than a physical ceiling, and an engine can be driven above 100 percent. Multi-spool engines also show separate N1 and N2 indications that are easy to mix up.

7 free sample questions from ACS AM.III.D

1. AM.III.DTap an answer

   A thermocouple-type cylinder head temperature (CHT) indicating system generates a voltage based on what principle?

   • AThe expansion of a liquid-filled bulb against a bourdon tube
   • BThe change in electrical resistance of a single metal coil
   • CThe temperature difference between two junctions of dissimilar metals
2. AM.III.DTap an answer

   A typical engine oil pressure gauge of the bourdon-tube type indicates pressure by which mechanical action?

   • AA curved tube tends to straighten as internal pressure increases
   • BA diaphragm expands against a calibrated electrical resistor
   • CA bimetallic strip deflects in proportion to applied heat
3. AM.III.DTap an answer

   On a turbine engine, exhaust gas temperature (EGT) is most commonly sensed using which type of device?

   • AA single bourdon-tube pressure element
   • BA bimetallic-spring dial assembly
   • CSeveral thermocouples wired in parallel
4. AM.III.DTap an answer

   A tachometer that uses a small AC generator on the engine driving a synchronous motor in the indicator is best described as which type of system?

   • AA mechanical flexible-drive cable system
   • BAn electric tachometer-generator system
   • CA thermocouple averaging system
5. AM.III.DTap an answer

   Turbine-engine tachometers are typically calibrated to indicate engine speed in what units?

   • APercent of maximum rated rpm
   • BPounds of thrust produced
   • CDegrees of exhaust gas temperature
6. AM.III.DTap an answer

   What does a manifold absolute pressure (MAP) gauge on a reciprocating engine primarily indicate?

   • AAbsolute pressure in the intake manifold
   • BDifferential pressure across the oil filter
   • CGauge pressure in the exhaust collector
7. AM.III.DTap an answer

   If a thermocouple lead in a CHT system is replaced, why must the resistance of the new leads match the value specified for that system?

   • AThe indicator is calibrated for a specific total lead resistance
   • BHigher resistance increases the thermocouple hot-junction temperature
   • CThe leads supply operating voltage to the indicator

Frequently asked questions

How does a thermocouple temperature system work without an external power source?

The thermocouple itself generates the signal. Joining two dissimilar metals creates a small voltage that varies with the temperature difference between the hot junction at the cylinder head or gas path and the reference junction at the indicator, and the indicator is really a sensitive voltmeter scaled in degrees. No aircraft power is needed for the sensing loop, which is why the gauge can still read with the master switch off.

What does a manifold pressure gauge actually measure?

It measures absolute pressure inside the intake manifold, normally in inches of mercury. On a normally aspirated engine, manifold pressure sits below ambient at idle and rises toward ambient as the throttle opens, and on a turbocharged engine it can exceed ambient. Combined with rpm, it is how the pilot sets and the mechanic verifies power output on a reciprocating engine.

Why must replacement thermocouple leads have the specified resistance?

Because the indicator is calibrated for one fixed value of external circuit resistance. The thermocouple output is a very small voltage, so any change in lead resistance changes the current the indicator sees and shifts the whole scale. Leads are supplied as matched sets of a set length and material, and coiling excess length is the accepted fix rather than cutting them shorter.

Why do turbine tachometers read in percent instead of rpm?

Because turbine rotor speeds are very high and differ from engine to engine, a percent scale gives every model a common frame of reference. The manufacturer designates a rated rpm as 100 percent, and the indicator shows engine speed against that reference, which makes limits, power settings, and checklist figures directly comparable across different engines and different spools.

What does Pt7 mean in gas turbine station notation?

It is the total pressure sensed at engine station 7. The letter P stands for pressure, the subscript t means total rather than static, and the number identifies the station along the gas path, with station 7 at the exhaust end of the engine. That reading is one of the inputs used to derive engine pressure ratio on engines that indicate thrust that way.

All 40 ACS areas

Primary sources

• FAA Mechanic Airman Certification Standards (FAA-S-ACS-1)
• 14 CFR Part 65 — Certification: Airmen Other Than Flight Crewmembers