Instruments

Flight instruments tell you what the aircraft is doing when your eyes and body cannot. A light aircraft has six basic flight instruments, the "six-pack", plus a magnetic compass and the engine gauges. Knowing how each one works also tells you how it fails – and how to spot the failure.

1. The six-pack

Top row	Bottom row
Airspeed indicator (ASI)	Turn coordinator
Attitude indicator (AI, artificial horizon)	Heading indicator (HI, directional gyro)
Altimeter	Vertical speed indicator (VSI)

The three pressure instruments (ASI, altimeter, VSI) work from the pitot-static system; the other three are gyroscopic.

2. The pitot-static system

• The pitot tube faces into the airflow and feels total pressure (static plus the pressure of the airflow).
• Static ports on the side of the fuselage feel the still-air static pressure.
• The ASI uses both: the difference is dynamic pressure, shown as airspeed.
• The altimeter and VSI use static pressure only.

Blockages

Blocked	ASI	Altimeter	VSI
Pitot tube (drain hole clear)	Drops to zero	Normal	Normal
Pitot tube and drain hole	Acts like an altimeter: reads higher as you climb, lower as you descend	Normal	Normal
Static ports	Under-reads in the climb, over-reads in the descent	Frozen	Stuck at zero

Ice is the usual cause, which is why there is pitot heat. If the static ports block, select the alternate static source (in the cabin, where pressure is slightly lower, so the altimeter and ASI read a little high).

3. Airspeed indicator

Marking	Meaning
White arc	Flap operating range: from VS0 (stall, flaps and gear down) to VFE (max flap extended speed)
Green arc	Normal operating range: from VS1 (stall, clean) to VNO (max structural cruising speed)
Yellow arc	Caution range: only in smooth air, from VNO to VNE
Red line	VNE: never exceed speed

4. Altimeter

The altimeter is a barometer: sealed capsules expand as pressure falls. The setting window (the "Kollsman window") must hold the right pressure – QNH below the transition altitude, 1013 hPa above it.

• Flying from high pressure to low pressure without resetting, the altimeter over-reads: you are lower than shown. "High to low, look out below."
• The same is true flying into colder air: in very cold weather you are lower than the altimeter says.
• 1 hPa is about 30 ft near sea level. A QNH 10 hPa wrong is a 300 ft error.

5. Vertical speed indicator

The VSI compares the static pressure now with the pressure a moment ago (through a small calibrated leak). It shows the rate of climb or descent in feet per minute, with a few seconds of lag – use it for trends, and the attitude indicator for the pitch itself.

6. Gyroscopic instruments

A spinning gyroscope has two properties:

• Rigidity in space: it keeps pointing the same way, which gives us a stable reference.
• Precession: a force applied to it acts 90° further round in the direction of rotation – the source of some gyro errors.

Gyros are spun by a vacuum pump (air-driven) or electrically. A suction gauge shows whether the vacuum system is working.

• Attitude indicator: shows pitch and bank against an artificial horizon. It is the master instrument of instrument flying.
• Heading indicator: shows heading without the compass's turning errors, but it slowly drifts – realign it with the compass about every 15 minutes in straight and level flight.
• Turn coordinator: shows the rate of turn. Wings on the "standard rate" mark means 3° per second – a full 360° in 2 minutes. The ball shows balance: if it is out to one side, "step on the ball" with that rudder pedal.

Rule of thumb: the bank for a standard-rate turn is about TAS ÷ 10 + 7 degrees (about 19° at 120 kt).

7. The magnetic compass

The compass needs no power and is the backup for everything else, but it has errors (northern hemisphere):

• Acceleration errors on east or west headings: ANDS – Accelerate North, Decelerate South. Accelerating, the compass swings towards north; decelerating, towards south.
• Turning errors: when turning onto north, the compass lags – roll out early (undershoot north); turning onto south it leads – roll out late (overshoot south). Remember UNOS.
• Read it only in steady, straight and level flight.

8. Engine instruments

• Tachometer (RPM) and, with a variable-pitch propeller, manifold pressure.
• Oil pressure (should rise within about 30 seconds of start) and oil temperature.
• Fuel quantity and fuel pressure; EGT for leaning the mixture.
• Engines in jets show N1/N2, EGT and fuel flow.

9. Glass cockpits

Modern aircraft show the same information on a PFD (primary flight display): attitude in the middle, speed tape on the left, altitude tape and vertical speed on the right, heading below. The sensors are an ADC (air data computer) and an AHRS (attitude and heading reference system) instead of mechanical gyros, but the principles – and the effect of a blocked pitot or static port – are the same.

10. The instrument scan

Use the attitude indicator as the hub: look at it, then out to another instrument and back – the selective radial scan. Set the attitude, let it settle, then check the performance instruments (altimeter, ASI, VSI, heading) and adjust.

Flying it at Levant

In the A320 or 737 you will use a PFD rather than a six-pack, but the same rules apply: check the altimeter setting at transition, cross-check the standby instruments, and if speeds start disagreeing in icing conditions, think of a blocked pitot tube. Many simulators let you fail instruments – practise flying on the standby instruments.

Check yourself

1. Which instruments use only static pressure? The altimeter and the VSI.
2. The static ports are blocked. What does the altimeter show? It freezes at the altitude where the blockage happened.
3. You fly from a high-pressure area into a low without changing the setting. Are you higher or lower than indicated? Lower – high to low, look out below.
4. How long does a standard-rate 360° turn take? 2 minutes (3° per second).
5. Turning onto north with the magnetic compass, do you roll out early or late? Early (undershoot north).

Simulation training material based on standard PPL instrument theory. Always follow your aircraft's flight manual.