Type Rating Knowledge — Cessna 172S NAV III

Pitot-static system and flight instruments

One pitot head, one static port, one alternate source — and then two computers that turn them into six indications. Learn which box feeds which needle and a failure stops being a puzzle: air data and attitude fail in two clean, separate halves.

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The pitot-static system

Two holes in the aeroplane

Everything in this module starts from two openings. Total pressure comes from a heated pitot head on the LOWER surface of the left wing — one head, with an electrical heating element built into its body, switched by PITOT HEAT on the panel below the lower left corner of the PFD. Static pressure comes from an external static port on the LEFT side of the forward fuselage. Plumbing connects both to the air data computer and to the conventional pitot-static instruments. That word "conventional" is doing real work: the standby airspeed indicator and standby altimeter on the lower centre panel are plumbed to the same two holes directly, with no computer in the path at all.

PITOT HEAD — Heated total-pressure head on the LOWER surface of the left wing. It is the only source of total pressure on the aeroplane.
OAT PROBE — Outside air temperature probe on top of the cabin, wired to the air data computer.
STATIC PORT — External static port on the LEFT side of the forward fuselage — the normal static source.
PITOT HEAT SW — Switch on the panel below the lower left corner of the PFD, feeding the heating element built into the body of the pitot head. Its breaker is on the lower left breaker panel.
ALT STATIC AIR — Alternate static source valve beside the throttle. Pulled on, it takes static pressure from INSIDE the cabin.
PITOT LINE — Plumbing carrying total pressure aft to the air data computer and to the standby airspeed indicator.
STATIC LINE — Plumbing carrying static pressure to the air data computer and to both standby pressure instruments.
AIR DATA COMPUTER (GDC) — Mounted behind the panel just forward of the MFD. It calculates pressure altitude, airspeed, true airspeed, vertical speed and outside air temperature.
STANDBY ASI — Pneumatic standby airspeed indicator on the lower centre panel, driven straight off the pitot and static lines with no computer in between.
STANDBY ALTIMETER — Aneroid sensitive standby altimeter on the lower centre panel, driven straight off the static line.
PFD AIRSPEED TAPE — Vertical tape airspeed indication on the upper left of the PFD, computed by the air data computer.
PFD ALTIMETER TAPE — Primary altitude tape right of the attitude indicator, computed by the air data computer. BARO is set with the knob on the GDU bezel.
PFD VSI TAPE — Vertical speed tape right of the altimeter. Digits appear inside the pointer only above 100 feet per minute.

Redrawn from POH 172SPHBUS §7, Pitot-Static System and Instruments / Air Data Computer (GDC) — structure restated, not traced. C172S NAV III.

Where is the pitot head?

On the LOWER surface of the LEFT wing, heated by an element built into its body. Right. Under the left wing, which is why it is easy to walk past on a preflight and easy to leave a cover on.

What does the ALT STATIC AIR valve give you, and where is it?

Static pressure from inside the cabin, from a valve adjacent to the throttle control. Right on both counts. It is not an outside backup port — it is the cabin, which is why it comes with a correction chart.

What the alternate static source costs you

Pull ALT STATIC AIR when you suspect water or ice in the line to the external port, and your static reference becomes the cabin. The POH is blunt about the consequence: pressures within the cabin vary with open heaters, vents and windows. So the indication error is not a fixed offset you can memorise — it depends on how the aeroplane is configured at that moment. Section 5, Figure 5-1 (Sheet 2) carries the Airspeed Calibration, Alternate Static Source correction chart, and using it means first knowing what the cabin is doing. That is the practical discipline: if you go to the alternate source, settle the cabin configuration and leave it settled, so the correction you are applying stays the correction you looked up.

Why does the POH give a correction chart for the alternate static source rather than a single number?

Because cabin pressure varies with open heaters, vents and windows, so the error depends on the aeroplane’s configuration. Right, and the practical consequence is to fix the cabin configuration and leave it fixed while you are on the alternate source.

Air data and the three tapes

The box in the middle

The air data computer — the GDC — sits behind the instrument panel just forward of the MFD, and it is what turns pressures into numbers. It compiles the pitot-static system and calculates pressure altitude, airspeed, TRUE airspeed, vertical speed and outside air temperature. That last one is why an outside air temperature probe on top of the cabin is wired into it: true airspeed needs temperature, so the OAT probe is an air-data input rather than a piece of cabin trivia. Everything the ADC produces appears on the PFD as a tape — a vertical scrolling scale with a fixed pointer, in place of a dial with a rotating needle.

Which of these does the air data computer calculate?

Pressure altitude, airspeed, true airspeed, vertical speed and outside air temperature. Right — five outputs from two pressures and a temperature probe. Note that true airspeed is in there, which is what the OAT probe is for.

You are descending at 60 feet per minute. What does the VSI pointer show?

The pointer moves, but no digits appear — vertical speed must exceed 100 ft/min before digits are displayed inside the pointer. Right, and it catches people out in the level-off: a blank pointer is not a broken VSI, it is a small vertical speed.

What drives each indication (POH 172SPHBUS §7, Flight Instruments / Standard Avionics)
Indication	Where it is	Driven by
Airspeed tape	Upper left of the PFD	Air data computer (GDC)
Altitude tape	Right of the attitude indicator	Air data computer (GDC)
Vertical speed tape	Right of the altimeter	Air data computer (GDC)
Attitude indicator	Upper centre of the PFD	AHRS (GRS)
Slip/skid trapezoid	Below the roll pointer	AHRS (GRS)
HSI	Lower centre of the PFD	AHRS and magnetometer
Standby airspeed indicator	Lower centre panel	Pitot and static lines directly (pneumatic)
Standby altimeter	Lower centre panel	Static line directly (aneroid)
Standby attitude indicator	Lower centre panel	Vacuum system

The air data computer fails. What happens to the standby airspeed indicator and standby altimeter?

Nothing — they are pneumatic and aneroid instruments plumbed straight to the pitot and static lines, with no computer in the path. Right, and this is the whole reason they are there. They fail with the plumbing, not with the electronics.

MAGNETOMETER — Magnetometer inside the LEFT wing panel. It interfaces with the AHRS to provide heading — it does not drive the HSI directly.
OAT PROBE — Air temperature sensor on top of the cabin, feeding the air data computer.
AHRS (GRS) — Attitude and heading reference system in the tailcone. Accelerometers, tilt sensors and rate sensors — no spinning mass gyros.
AIR DATA COMPUTER — GDC air data computer behind the panel, compiling the pitot-static system into airspeed, altitude, vertical speed, TAS and OAT.
ATTITUDE (AI) — Upper centre of the PFD. Horizon line the full width of the display; roll scale 10° to 30° then 15° to 60°; pitch marked every 5°, labelled every 10°; red chevrons point back to level if pitch limits are exceeded.
SLIP/SKID TRAPEZOID — Small white trapezoid below the roll pointer, moving laterally. It replaces the skid indicator ball, and centred is coordinated.
AIRSPEED TAPE — Vertical tape with fixed pointer, upper left. Coloured bands for maximum speed, high cruise caution, normal operating, full flap and low airspeed awareness; TAS in a window at the bottom.
ALTITUDE TAPE — Primary altimeter right of the attitude indicator. BARO knob sets local pressure; the cyan bug is set with ALT SEL and read in a window at the top.
HSI — Lower centre of the PFD. Stabilised compass card with selectable GPS or VHF deviation, 45° reference marks, a turn-rate segment scale, a cyan HDG bug and a CRS pointer.
VERTICAL SPEED — VSI tape right of the altimeter, digits inside the moving pointer. Nothing appears in the pointer below 100 ft/min.

Redrawn from POH 172SPHBUS §7, Flight Instruments (Attitude Indicator, Airspeed Indicator, Altimeter, HSI, VSI) and Standard Avionics (AHRS/magnetometer, GDC) — structure restated, not traced.

Attitude and heading

Attitude, and what the trapezoid replaced

The attitude indicator sits on the upper centre of the PFD, drawn from the AHRS in the tailcone — accelerometers, tilt sensors and rate sensors, with no spinning mass gyro anywhere in it. The horizon line runs the full width of the display. The roll index is conventional but not evenly spaced: 10° graduations out to 30°, then 15° graduations out to 60°. Pitch is marked every 5° and labelled every 10°, and if pitch limits are exceeded in either direction red warning chevrons appear pointing the way back to level. Below the roll pointer is a small white trapezoid that slides left and right. That is the slip/skid indication — it does the job the ball in the inclinometer used to do, and centred beneath the roll pointer is coordinated.

How is the roll index scale graduated?

10° graduations out to 30°, then 15° graduations out to 60°. Right — finer where you live, coarser where you should not be. Reading 45° as "one mark past 30" is the practical consequence.

What is the small white trapezoid below the roll pointer?

The slip/skid indication — it replaces the skid indicator ball, and should be centred below the roll pointer for coordinated turns. Right. Same information as the ball, moved into the attitude display so your scan does not have to leave it.

Heading, and the box it really comes from

The HSI occupies the lower centre of the PFD and behaves like the instrument it is named after: a stabilised magnetic compass card with selectable navigation deviation on top of it. Magnetic heading appears numerically above the lubber line; reference index marks sit at 45° intervals around the card; a circular segment scale below the heading window shows half and standard rate turns by the length of a magenta vector. The cyan HDG bug is set with the HDG knob and reads out above the upper left 45° mark; the course pointer is set with CRS and reads out above the upper right one. The CDI softkey selects what the deviation needle is following — GPS, NAV 1 or NAV 2. But the heading itself does not come from a compass in the panel. It comes from the AHRS, which takes its magnetic reference from a MAGNETOMETER buried inside the left wing panel, far from the steel and the current in the cockpit.

Where is the magnetometer, and what does it feed?

Inside the left wing panel; it interfaces with the AHRS, which is what actually drives the HSI. Right. Out in the wing, away from the ferrous metal and wiring of the cockpit — and it feeds the AHRS rather than the display directly.

On the anchor NAV III aeroplane, what does an engine-driven vacuum pump failure cost you?

The standby attitude indicator only — the PFD attitude and the HSI come from the AHRS and are unaffected. Right. The NAV III kept the vacuum system, but narrowed its job to one instrument. That is the opposite of the steam-gauge case.