12.3 Engine Instruments, Transponder Modes, and Basic Avionics

ACS PA.I.G.K1g (avionics) and PA.I.G.K1h (pitot-static, vacuum/pressure, and associated instruments) sit next to the engine-instrument knowledge in PA.I.G.K1e. The knowledge test writes them as separate stems: an oil-pressure needle that never rises, a transponder left in standby inside the Mode C veil, a 7600 versus 7700 mix-up, or a PFD red X that students treat as “the whole airplane failed.”

Engine instruments the written actually samples

Oil pressure is the first gauge after the propeller turns. No pressure within the time the POH allows — often on the order of thirty seconds, but the book for that engine wins — and you shut down on the ramp. Pressure is a restriction and a pump, not a weather report. Low pressure plus rising oil temperature is an engine that is about to stop making metal chips you cannot afford. A sudden zero with a still-normal temperature can be a gauge or a line; you still treat it as a landing problem, not a “fly it off” problem.

Oil temperature lags. It is a heat story: mixture, climb attitude, cowl flaps if you have them, outside air, and how hard you are working the engine. High oil temp with good pressure is cooling and power management. Do not lean or climb harder to “fix” a dry-pressure needle.

Cylinder head temperature (CHT) is how hot the metal at the head is. High CHT is a detonation and valve-life problem: too much power, too lean, too little cooling air, or a cowl flap left closed. Exhaust gas temperature (EGT) is the mixture instrument. You lean toward peak EGT and then follow the POH — many trainers are run on the rich side of peak in cruise. EGT is not a substitute for CHT; a cool pipe can still sit on a hot head if a baffle is missing.

Tachometer is RPM. On a fixed-pitch propeller, RPM and power live on the same lever: more throttle, more RPM, more power, until the redline. On a constant-speed propeller, the propeller control sets RPM and the throttle sets manifold pressure (MAP). MAP is the absolute pressure in the intake manifold. Near sea level, full throttle is close to ambient (high twenties of inches of mercury); a closed throttle at idle is a low MAP. “MAP is power, RPM is how fast the blades are turning” is the PAR sentence. Do not run a high MAP at a low RPM just because both needles are still in a green arc you did not read in the POH.

Fuel gauges are required by 14 CFR 91.205(b) — a gauge showing quantity in each tank — but PHAK and every honest POH say they are not precision instruments. A visual check during preflight is the quantity method that keeps you out of a 91.13 story. A gauge that sits on 1/4 while the tank is empty is a classic PAR and ramp fact pattern.

Day-VFR 91.205(b) also puts the ASI, altimeter, magnetic direction indicator, tachometer, oil pressure, oil temperature (air-cooled), and MAP (altitude engine / constant-speed installations) on the legal panel. That list is the reason these needles exist on the written. It is not a second airworthiness chapter.

Transponder modes, ident, and the codes you must not shuffle

A transponder is an ATC beacon. It does not navigate. It replies when a radar or ADS-B ground station interrogates it.

Typical trainer control positions are OFF, SBY (warm, not replying), ON (Mode A identity, no altitude), ALT (Mode C or S with altitude), and TST. 91.215 wants ALT — identity and altitude — when the box is installed and you are in the airspace the rule names, including all controlled airspace. Leaving the unit in ON (no altitude) or SBY under a Class B shelf is how you write a deviation.

Ident is a button, not a code. When ATC says “squawk ident,” you press IDENT. The reply highlights on the scope for a short interval so the controller can pick you out of a pile of 1200s. You do not change the code to “ident.” The reply light blinking in normal flight is interrogations being answered; a steady ident flash is the button you just pressed.

Say them in order so you never swap 76 and 77. 7500 is the one you do not “try out” on a ground-check. If you have a radio and an emergency, you can still say the problem and squawk 7700. If the radios are dead, 7600 is the lost-comm code; the AIM lost-comm procedure is a different chapter. 91.215 replies: set the assigned code or 1200, operate in ALT, ident when asked, and if the transponder fails in airspace that requires one, ask ATC for a deviation under 91.215(d) rather than burrowing through the veil in standby.

The where of Mode C — Class A/B/C, the 30 NM veil, above B/C to 10,000 inside the lateral limits, and at or above 10,000 MSL excluding at or below 2,500 AGL — is taught with the rest of 91.215 in Chapter 4. This section is the box and the codes.

ADS-B Out versus In

14 CFR 91.225 is ADS-B Out: the airplane broadcasts GPS position, velocity, and identity so ATC and other equipped aircraft can see you without waiting for a radar sweep. The airspace is almost the same list as 91.215, plus the Gulf of Mexico Class E shelf. Class A needs 1090ES. Below 18,000 MSL, 1090ES or 978 UAT satisfies the equipment rule. If Out is installed, 91.225(f) says keep it transmitting unless the FAA or ATC authorizes otherwise.

ADS-B In is a receiver: traffic (TIS-B / ADS-R) and, on 978, FIS-B weather on a tablet or MFD. In is not what 91.225 requires. A portable In receiver does not make an unequipped airframe legal in a Mode C veil. Out is the mandate; In is the nice display.

VOR/COM preview — not the navigation chapter

The stack in front of you is usually a COM radio (118–136 MHz class, two-way voice) and a NAV receiver (VOR on 108.0–117.95 MHz). You tune COM to talk; you tune NAV to a VOR, identify the Morse, set the OBS, and read a CDI with a TO/FROM flag. That is enough to know which knob is not the transponder. Intercepting radials, VOR service volumes, and “which way does the CDI go on the back course” belong in the navigation chapter. Do not invent a PAR rule that a VOR identifies itself with a voice only, or that 120.0 is a VOR frequency.

Glass: PFD, AHRS, ADC — fail the computer, not “the sky”

A primary flight display (PFD) paints the same three pitot-static stories and the same two gyro stories on one screen.

• AHRS (attitude and heading reference system) is the solid-state stand-in for the vacuum AI and HI: gyros or MEMS rates, accelerometers, and usually a magnetometer. AHRS failure typically red-Xes attitude and often heading. Use the backup attitude indicator if the airplane has one. Do not follow a red-X horizon.
• ADC (air data computer) is the stand-in for the ASI, altimeter, and VSI. It still drinks the pitot-static system. ADC failure, or the same ice that blocks a steam gauge, red-Xes airspeed, altitude, and VSI. The blockage table in 12.1 still applies — a blocked pitot does not freeze the tape because the screen is glass; it freezes or lies because the plumbing is wrong.
• A magnetometer failure can take heading without taking attitude.
• A display failure is not an AHRS failure: many systems revert the remaining screen to a combined PFD (reversionary mode).
• An electrical failure still darkens a glass panel once the battery or backup path is gone. A vacuum backup AI, if installed, is the same gift it was in 12.2: it does not care that the PFD just died.

Pitot heat remains the anti-ice for the ram inlet the ADC uses. Turn it on before the red X, not after. It is still not a 91.205 day-VFR required item; it is how you keep 12.1 from becoming your afternoon.

Scenario: Nadia’s 7600 and a red X

Nadia is VFR into a Class C. She is in ALT, squawking the code departure gave her, and she knows ident is a button. The COM dies. She does not squawk 7700 for a radio that quit in VMC; she squawks 7600, keeps flying the clearance she has, and uses light-gun signals if she has to come home to a tower. Crossing the next ridgeline the PFD paints a red X over airspeed, altitude, and VSI while the horizon is still alive. That is ADC / pitot-static, not AHRS. She looks at the backup ASI and altimeter, checks pitot heat, and does not roll into a partial-panel unusual attitude she does not have. If the red X had been on the attitude tape with airspeed still honest, she would have been in the AHRS failure, flying the backup horizon and the same 12.2 scan her steam-gauge instructor started with.