Run-up checks

Run-up Checks

Run-up is the last complete engine and system check before take-off, done at a designated run-up position. Mandatory for all powered aircraft per POH.

Position

• Run-up bay off the active runway.
• Nose into wind (for engine cooling and ATC conformity).
• Brakes set, nose on firm ground (not on gravel).

Standard run-up — typical sequence (Cessna 172 example; POH-specific)

1. Engine warm-up

• Throttle to 1000–1200 RPM.
• Wait for oil temperature in the green (typ. ≥ 40 °C, per POH).

2. Mag check (ignition circuit test)

• Throttle to run-up RPM (POH; often 1700 RPM C172, 2000 PA-28).
• Both → Right → Both (RPM drop noted, should be < 175 RPM, no stumble).
• Both → Left → Both (same test).
• Difference Right ↔ Left should be < 50 RPM.
• If rough running or drop > 175 RPM: magneto defect — cancel flight.

Special: rough running on mag check at a high-altitude aerodrome

At a high-altitude airfield (pressure altitude > ~3000 ft) the engine runs with an over-rich mixture due to lower air density — the mag check can be rougher than at sea level. If the engine continues to run rough after the mag check and carb heat doesn't help:

• Check whether mixture leaning helps — pull the mixture back slightly, then repeat the mag check. At altitude this normalises the run.
• If leaning cures the rough run: the over-rich mixture was the cause, take off with leaned mixture (POH high-altitude setting).

3. Carb heat check (carbureted engines)

• Carb heat ON → RPM drop should match the value specified in the flight manual (POH) (typ. 50–100 RPM for C172). If the RPM drop is in the expected range, the heating works as intended.
• Carb heat OFF afterwards.
• Carb heat stays OFF for take-off (see below) — even when the weather suggests carburettor icing.

4. Propeller (constant speed)

• Throttle to 2000 RPM (POH).
• Prop lever slowly back → RPM must fall (typ. 300–500 RPM).
• Prop lever forward again → RPM back to 2000.
• Cycle several times to circulate oil.

5. Vacuum / suction gauge

• In the green (typ. 4.8–5.2 inHg) — indicates working gyro instruments (AI, DG).

6. Ammeter / voltmeter

• Battery being charged by alternator → ammeter shows + ; voltmeter in green (typ. 13.5–14.8 V).

7. Engine gauges

• Oil pressure in green.
• Oil temperature in normal range (rises with warm-up).
• CHT (if equipped) ok.
• EGT ok.

8. Controls

• Control surfaces full and free: yoke fully left/right/forward/back, pedals full.

9. Trim

• Trim to TAKE-OFF position (see POH; usually neutral or slightly nose-up).

10. Flaps

• Flaps to 10° (often for take-off, POH-dependent).

11. Pre-take-off briefing

• Take-off procedure spoken: speeds Vr, V2, "rotation at 55 KIAS, climb at 75 KIAS to 500 ft AGL, then Vy 79".
• Engine failure plan: "On power loss before Vr: stop on runway. After Vr and < 500 AGL: straight or slight turn, landing. Above 500 AGL: pick a field."

Carburettor icing — background

Carburettor icing can occur within an outside air temperature range of about -5 °C to +20 °C with high humidity (> 50 % relative). This is a counter-intuitive range, because:

• Ice at plus temperatures sounds impossible, but is realistic due to the pressure and temperature drop in the carburettor.
• Inside the carburettor, the temperature drops by up to 20–30 °C through Bernoulli effect and fuel evaporation — at +15 °C outside the carburettor interior can be -10 °C, where water vapour freezes.

Even when meteorological conditions suggest carb icing, carb heat is still switched OFF during take-off ("carb heat OFF for take-off"). Reasons:

• Full engine power is vital for take-off; carb heat reduces power by about 10 %.
• Unfiltered air via carb heat (see lesson "Taxiing") is especially risky near the runway.
• Carb heat is reactivated in cruise and especially in descent / approach.

→ The pilot must monitor for carburettor icing in cruise and approach via carb heat.

Manual propeller rotation — critical rule

If the magneto switch (ignition) is in "BOTH" and the master switch is OFF, the engine can start when the propeller is moved manually. Reason: the magnetos generate ignition voltage independently of the master switch and aircraft electrical power — they are coupled directly to the engine and propeller rotation.

→ Consequence:

• Never rotate the propeller by hand without verifying magnetos OFF (ignition switch OFF).
• Before maintenance, cleaning, or inspection of the propeller: ignition switch OFF, key removed.
• Master switch alone is not enough — it only disconnects the electrical system, not the ignition.

Propeller turned backward after failed start

If an engine start was unsuccessful (e.g. flooded engine from too much priming), the mechanic or pilot can rotate the propeller in the reverse direction, because:

• The engine has received too much fuel (flooded carburettor / cylinder).
• Rotating backward draws fresh air through the outlet valve into the cylinders, diluting the over-rich mixture.
• A normal restart is then possible.

Safety rule: only perform with magnetos OFF (see above), and a pilot in the cockpit for brake control.

What not to check?

• Avionics during run-up only briefly to save power — avionics master usually on only after run-up.
• Radio frequencies already set before run-up (tower, departure).

POH governance

All values and order are POH-specific. This lesson gives typical examples from the Cessna 172 POH. Other aircraft (PA-28, DA-40, DR400, Aquila) have different values and order.