Type Rating Knowledge — Cessna 172S NAV III

Avionics — G1000 and the GFC 700 AFCS

Not a button course. Which line replaceable unit each function lives in, because that is what decides what a failure takes with it — and the two warnings the POH prints twice: the fans that stop on standby battery power, and the autopilot that drops to wings-level with no aural alert.

Take the interactive lesson →

The displays and reversionary mode

Two identical screens doing two different jobs

The G1000 puts two IDENTICAL display units on the panel. The one in front of the pilot is configured as the PFD; the one to the right is configured as the MFD. Identical is the important word — they are the same part number doing different jobs because of how they are configured, not because one is a flight instrument and the other is a map. The PFD carries roll and pitch, heading and course, altitude, airspeed and vertical speed, and it also controls and displays every communication and navigation frequency and the aeroplane’s warning and status annunciations. The MFD carries a large scalable moving map with traffic, lightning and weather overlaid on it, and it is the principal display for all engine, fuel and electrical parameters.

AHRS (GRS) — Attitude and heading reference system in the tailcone — attitude and flight characteristics.
MAGNETOMETER — In the left wing panel; interfaces with the AHRS to provide heading.
ADC (GDC) — Air data computer behind the panel, forward of the MFD, with the OAT probe wired to it.
ENGINE MON (GEA) — Engine monitor receiving every engine and airframe sensor — CHT, EGT, RPM, fuel flow and the fuel gauging system — and transmitting to the engine display computers.
XPDR (GTX) — Mode S transponder in the tailcone avionics racks, providing Mode A, C and S. It is controlled from the PFD, not from a box of its own.
XM DATALINK (GDL) — GDL-69A XM weather and radio data link in the tailcone, on the high-speed bus to the MFD. A subscription is required for it to be used.
GIA 1 — Integrated avionics unit 1, in a tailcone rack. Communications hub linking the peripherals to the displays, and it CONTAINS a GPS receiver, a VHF nav receiver, a VHF comm transceiver and the main system microprocessors.
GIA 2 — Integrated avionics unit 2, identical to GIA 1. The FIRST unit to acquire a GPS 3-D navigation signal is the active GPS source — not a fixed priority.
PFD (GDU) — Display unit in front of the pilot, configured as the primary flight display. It also controls and displays all comm and nav frequencies and the airplane system annunciations.
AUDIO PANEL (GMA) — Between the two displays. It integrates the comm and nav digital audio, the intercom and the marker beacon — and it is the unit that controls REVERSIONARY mode for both displays.
MFD (GDU) — Identical display unit to the right, configured as the multi-function display: scalable moving map, traffic, lightning and weather overlays, and the principal display for engine, fuel and electrical parameters.
COM 1 — VHF communication transceiver inside GIA 1.
NAV 1 — VHF navigation receiver inside GIA 1, fed by a blade antenna on the vertical stabilizer.
GPS 1 — GPS receiver inside GIA 1, fed by the COM 1/GPS 1 dual-mode antenna on the right of the cabin roof.
COM 2 — VHF communication transceiver inside GIA 2.
NAV 2 — VHF navigation receiver inside GIA 2, fed by the blade antenna on the other side of the fin.
GPS 2 — GPS receiver inside GIA 2, fed by the COM 2/GPS 2 dual-mode antenna on the left of the cabin roof.

Redrawn from POH 172SPHBUS §7, Standard Avionics (GDU, GMA, GIA, GRS/magnetometer, GDC, GEA, GTX, GDL) — structure restated, not traced. C172S NAV III.

Where are the engine, fuel and electrical parameters normally displayed?

On the MFD — it is the principal display for all of them. Right. The PFD carries the flight instruments and the frequencies; the systems picture lives on the MFD in normal operation.

What does reversionary mode do?

Places the flight information and basic engine information on BOTH the PFD and the MFD, so a working screen carries everything necessary. Right. It is a display configuration change, not a degraded mode — the surviving screen shows what you need to fly.

What that means for a display failure

Because the two units are identical and reversionary mode exists, the failure of one display is a workload event rather than a loss of information. The pilot keeps full access to everything necessary — flight information plus basic engine information — on the one that is left. What you lose is the luxury of a full moving map beside a full flight display, and you lose the second copy of the screen you are now depending on. That is worth planning around in a way "the PFD failed" does not immediately suggest: the aeroplane is now single-display, and the button that got you there lives on the audio panel.

The PFD goes blank in cloud. What is your immediate resource?

Reversionary mode from the audio panel — the MFD then shows flight information plus basic engine information. Right, and the standby attitude indicator, airspeed indicator and altimeter are there throughout as the layer underneath that.

Audio panel and the integrated avionics units

The audio panel, and one line to remember

The audio panel — the GMA — sits between the two displays and integrates all the communication and navigation digital audio, the intercom system and the marker beacon controls in a single unit. It is more than an audio selector: it also owns reversionary mode, as you have just seen. And the POH prints one flat prohibition about it, in a NOTE with nothing else in it: use of the COM 1/2 function is not approved. That is a limitation on this installation, not a general statement about audio panels, and it is the kind of line that only exists in the aeroplane’s own book.

What does the audio panel integrate?

All the communication and navigation digital audio, the intercom system and the marker beacon controls. Right — and, less obviously, the reversionary mode control for both displays.

What does the POH say about the COM 1/2 function on the audio panel?

Its use is not approved. Right, flatly and without elaboration. An installation-specific limitation you would never guess from knowing audio panels in general.

The two boxes everything goes through

Behind the baggage curtain, in racks in the tailcone, sit two integrated avionics units. They act as the main communications hub linking every peripheral part to the displays — and each one CONTAINS a GPS receiver, a VHF navigation receiver, a VHF communication transceiver and the main system microprocessors. That containment is the whole reason to learn the architecture. "COM 1", "NAV 1" and "GPS 1" are not three boxes; they are three functions of one box, so they live and die together. And which GPS is in charge is not fixed by wiring: the FIRST unit to acquire a GPS satellite 3-D navigation signal is the active GPS source.

What does each integrated avionics unit contain?

A GPS receiver, a VHF navigation receiver, a VHF communication transceiver, and the main system microprocessors. Right — four things in one box, twice over. This is why a GIA failure is a grouped loss rather than a single-function one.

Which GPS receiver is the active source?

Whichever unit acquires a 3-D navigation signal first. Right — the POH says so explicitly. It is not a fixed priority, so "GPS 1 is primary" is not a fact about this system.

Sensors, engine monitor, transponder and data link

The sensors, and where they live

Around the two hub units sit the boxes that measure things. The AHRS is in the tailcone — accelerometers, tilt sensors and rate sensors replacing spinning mass gyros — and it takes its magnetic reference from a magnetometer inside the LEFT WING PANEL. The air data computer is behind the instrument panel just forward of the MFD, with the outside air temperature probe on top of the cabin wired to it. The engine monitor receives and processes the signals from every engine and airframe sensor: all the cylinder head temperature sensors, the exhaust gas temperature sensors, RPM, fuel flow and the fuel gauging system. The Mode S transponder is in the tailcone avionics racks, and it is operated from the PFD — there is no transponder head on the panel. And the XM data link unit is in the tailcone behind the baggage curtain, talking to the MFD on the high-speed data bus.

What is inside the AHRS, and where is it?

Accelerometers, tilt sensors and rate sensors, in the tailcone — no spinning mass gyros. Right. Solid state and out of sight, which is why nothing on the panel needs to precess, erect or topple.

Why is the magnetometer in the left wing rather than behind the panel?

To get the magnetic sensor away from the ferrous metal and electrical current of the cockpit, where a magnetic reading would be corrupted. Right. It interfaces with the AHRS to provide heading, and it can only do that from somewhere magnetically quiet.

G1000 line replaceable units (POH 172SPHBUS §7, Standard Avionics)
Unit	Where	What it does
GDU ×2	Instrument panel	Two identical displays, configured as PFD and MFD
GMA	Panel, between the displays	Comm/nav audio, intercom, marker beacon — and reversionary mode control
GIA ×2	Tailcone racks, behind the baggage curtain	Communications hub; each contains GPS, VHF nav, VHF comm and the main microprocessors
GRS (AHRS)	Tailcone	Attitude and heading — accelerometers, tilt and rate sensors
Magnetometer	Inside the left wing panel	Magnetic reference, interfaced to the AHRS
GDC (ADC)	Behind the panel, forward of the MFD	Pressure altitude, airspeed, TAS, vertical speed, OAT
GEA	Engine/airframe sensor interface	All CHT and EGT sensors, RPM, fuel flow, fuel gauging
GTX	Tailcone avionics racks	Mode S transponder, Modes A/C/S — controlled from the PFD
GDL	Tailcone, behind the baggage curtain	XM weather and radio, to the MFD on the high-speed bus; subscription required

The engine monitor is connected to which sensors?

All the CHT sensors, the EGT sensors, RPM, fuel flow and the fuel gauging system. Right — which is why the fuel QUANTITY indication is downstream of the engine monitor, not of anything in the fuel system itself.

How is the transponder operated?

From the PFD — the unit itself is in the tailcone avionics racks with no head on the panel. Right. Code entry and mode selection are PFD functions, which is one more thing a PFD failure changes the workflow for.

GFC 700 AFCS and control wheel steering

An autopilot with no box of its own

The GFC 700 is an automatic flight control system integrated into the G1000 rather than bolted alongside it. There is no separate autopilot head on the panel: the mode keys and knobs are on the display bezels, and the computation lives in the integrated avionics units. Three servos do the flying — roll, pitch and pitch trim — and they drive the same aileron cables, elevator cables and elevator trim tab that the control wheel and trim wheel drive. On the pilot’s control wheel is the Control Wheel Steering button. Pressing it IMMEDIATELY disconnects the pitch and roll servos, so you hand-fly with the modes still engaged. It does not disconnect the pitch TRIM servo, and that asymmetry is the source of the one warning the POH attaches to it.

AFCS CONTROLS — Mode keys and knobs on the GDU bezel. The GFC 700 has no separate control box on the panel — the autopilot is part of the G1000, not a Bendix/King unit bolted next to it.
HSI NAV SOURCE — Whichever of GPS, NAV 1 or NAV 2 the CDI softkey has selected on the HSI. In NAV, APR and BC modes the autopilot steers to THIS, so changing it changes what the autopilot is following.
GIA — AFCS COMPUTATION — The autopilot computation lives in the integrated avionics units, which is why the GFC 700 is described as integrated rather than as an accessory.
ROLL SERVO — Roll servo, driving the aileron system.
PITCH SERVO — Pitch servo, driving the elevator system.
PITCH TRIM SERVO — Pitch trim servo, driving the elevator trim tab. It is NOT one of the two the CWS button disconnects.
CWS BUTTON — Control Wheel Steering button on the pilot’s control wheel. Pressing it immediately disconnects the pitch and roll servos so the aeroplane can be hand-flown.
AILERONS — Ailerons, moved by the roll servo through the same cables the control wheel uses.
ELEVATOR — Elevator, moved by the pitch servo through the same cables the control wheel uses.
ELEVATOR TRIM TAB — The single elevator trim tab — the same tab the manual trim wheel drives. Autotrim and hand trim are two ways to move one surface.

Redrawn from POH 172SPHBUS §7, GFC 700 Automatic Flight Control System (Figure 7-10) and Control Wheel Steering, with the HSI navigation-source warning from §7 Flight Instruments — structure restated, not traced.

Where does the GFC 700’s computation actually happen?

In the integrated avionics units — there is no separate autopilot computer or panel head. Right, and it is what "integrated" means here. The mode controls are on the display bezels, and the boxes doing the work are the same ones doing the navigation.

You make a large pitch change while holding CWS. What does the POH tell you to expect, and to do?

The aeroplane will be out of trim — retrim as necessary during CWS operation, or expect control forces and large pitch oscillations after you release the button. Right, in the POH’s own terms. The oscillations are the reason it is a warning rather than a note.

The warning the POH prints twice

One warning appears in §7 under the HSI and again under Control Wheel Steering, which is the book telling you it matters. With the autopilot engaged in NAV, APR or BC, manually changing the HSI navigation source with the CDI softkey INTERRUPTS the navigation signal to the autopilot and causes it to revert to ROL mode. NO AURAL ALERT IS PROVIDED. In ROL the autopilot only keeps the wings level — it will not correct heading and it will not correct course. So the aeroplane goes on flying, straight and level and wrong, off whatever heading it happened to be on, with nothing to tell you. The remedy is ordering: set the HDG bug to the correct heading and select the correct navigation source on the HSI BEFORE engaging the autopilot in any other mode.

You are in NAV mode on a GPS course and press the CDI softkey to look at NAV 1. What happens?

The autopilot reverts to ROL — wings level only, no heading or course correction — with no aural alert. Right, and the silence is the dangerous half. The aeroplane keeps flying beautifully in a direction nobody chose.

A NAV III is not automatically a GFC 700

Between 2005 and 2007 Cessna delivered NAV III 172S aeroplanes with the G1000 and a Bendix/King KAP 140 autopilot — a separate box on the panel, from a different manufacturer, not integrated into the G1000 at all. Same glass, same aeroplane name, different autopilot with different modes, different controls and no shared computation. Everything in this unit — the servos disconnected by CWS, the ROL reversion on a CDI source change, the mode keys on the display bezels — is a GFC 700 fact. The serial break is what tells you which you are in: the GFC 700 block starts at 172S10468, 172S10507, 172S10640 and 172S10656 and on, with POH 172SPHBUS. If the panel has an autopilot control box of its own, you are in the other aeroplane and you need 172SPHAUS.

You climb into a G1000-equipped 172S and there is a separate autopilot control box on the panel. What have you got?

The KAP 140 NAV III block — a different autopilot with its own POH, so nothing you know about the GFC 700 transfers. Right, and the separate box is the visible tell. Its POH is 172SPHAUS.

Avionics support equipment

What keeps the avionics alive and cool

Four DC electric fans cool the G1000 installation: one in the tailcone forcing air over the integrated avionics units and the transponder, one forward of the instrument panel pulling air from between the firewall bulkhead and the panel and directing it up the inside of the windshield, and two blowing onto the heat sinks on the forward faces of the PFD and MFD. Power reaches them only when the MASTER (BAT) switch AND the AVIONICS BUS 1 and BUS 2 switches are ALL on — three switches, not any one of them. And there is a NOTE with a real operational consequence: none of the cooling fans will operate when the essential bus avionics equipment is being powered by the standby battery.

MASTER (BAT) — The battery half of the split master switch.
AVIONICS BUS 1 & 2 — Both avionics switches. Cooling fan power needs the MASTER (BAT) switch AND both AVIONICS switches on — all three, not any of them.
AVIONICS COOLING FANS — Four DC electric fans providing forced and ambient air circulation for the G1000 equipment.
TAILCONE FAN — One fan in the tailcone, forced-air cooling the integrated avionics units and the transponder.
PANEL FAN — A fan forward of the instrument panel, pulling air from between the firewall bulkhead and the panel and directing it up the inside of the windshield.
PFD HEAT SINK FAN — A fan blowing directly onto the heat sink on the forward side of the PFD.
MFD HEAT SINK FAN — A fan blowing directly onto the heat sink on the forward side of the MFD.
STANDBY BATTERY — Drawn UNCONNECTED on purpose. The POH is explicit: none of the cooling fans operate when the essential bus avionics are being powered by the standby battery. The standby battery keeps avionics alive, not cool.
CABIN PWR 12V — Switch on the switch panel controlling the 12 volt cabin power outlet.
12 V CONVERTER — Power converter on the cabin side of the firewall, forward of the right instrument panel, reducing the airplane’s 28 VDC to 12 VDC.
STATIC DISCHARGERS — Wicks at various points on the airframe, reducing precipitation-static interference. Unpowered, and they lose effectiveness with age — checked at least at every annual.
POWER OUTLET 12V-10A — Outlet on the centre pedestal, limited to a maximum of 10 amps. The POH warns that charging lithium batteries may cause them to explode.
AUX AUDIO IN — Auxiliary audio input jack on the centre pedestal. It has NO switch — the AUX key on the audio panel does not control it, so the only way to deselect it is to unplug the cable.
AUDIO PANEL — The GMA. It automatically mutes the auxiliary audio during radio communications and during crew intercom isolation.

Redrawn from POH 172SPHBUS §7, Avionics Support Equipment — Avionics Cooling Fans, Auxiliary Audio Input Jack, 12V Power Outlet and Static Dischargers — structure restated, not traced.

What has to be on for the avionics cooling fans to run?

The MASTER (BAT) switch and BOTH avionics switches — BUS 1 and BUS 2. Right, all three. Selecting only one avionics bus leaves the fans off even though half the avionics are powered.

What is the limit on the 12 volt cabin power outlet, and what controls it?

A maximum of 10 amps, from a converter that steps the aeroplane’s 28 VDC down to 12 VDC, switched by CABIN PWR 12V on the switch panel. Right on all three. The outlet is even placarded POWER OUTLET 12V -10A, and the POH warns that charging lithium batteries may cause them to explode.

Two odd corners worth knowing

The auxiliary audio input jack on the centre pedestal lets a passenger play music over the headsets, and it has a genuine trap in it: it is not controlled by a switch. The signal mutes automatically during radio communications and during crew intercom isolation, but the AUX key on the audio panel does NOT control it, so the only way to deselect entertainment audio is to physically disconnect the source at the jack. The POH advises doing exactly that in high workload or heavy traffic. Separately, static dischargers are fitted at various points on the airframe to reduce precipitation static interference. They are unpowered, they lose effectiveness with age, and they should be checked at least at every annual inspection by a qualified technician — and even with them fitted, loss of radio signals is possible in severe static conditions, so the advice is to avoid known severe precipitation and to expect temporary losses if you cannot.

A passenger’s music is distracting you in a busy circuit. How do you turn it off?

Disconnect the cable at the AUX AUDIO IN jack — there is no switch, and the AUX key on the audio panel does not control it. Right, and the POH says as much: it is wise to disable the entertainment audio in high workload or heavy traffic, and unplugging is the only way.

What do static dischargers do, and what should you still expect?

They reduce precipitation-static interference — but loss of radio signals is still possible in severe static conditions, so avoid known severe precipitation and expect temporary losses. Right. They are a mitigation, not a cure, and they also lose effectiveness with age, which is why they are checked at least at every annual.