The Run-Up is something every pilot is taught to perform in the hold pad right before takeoff, but it is seldom discussed in detail, or worse, quick and incomplete information is given as to what the purpose is for each check.
Generally, the before flight checks can be grouped into several areas or events: The first pre-flight check is the documents/compass card check (interior). The second is the walkaround checking lights, fuel and overall aircraft condition (exterior). The third is the engine startup, oil pressure check and check of the gyro instruments and compass movement as the engine warms up during taxi (engine start and electronics). Lastly is the titular ‘Run-Up’ in the hold pad (engine stress).
Typical run-up tasks are as follows:
Engine Stress/Health Test: Bring the engine up to the manufacturer’s specified RPM setting, typically 1,700 or 1,800 or 2,000 RPM. This should bring the engine gauges into their green range and, at higher altitudes, be the proper time to lean the engine mixture for peak power. This is also a good time to check for any vibration, smooth engine operation (not missing, sputtering or stumbling), smoke, and no strange smells such as excessive exhaust fumes or the smell of burning oil or plastic.
Brake Stability and Reliability: Actively holding the brakes during the Run-Up is a good test to see if there is reliable pressure for the brakes and the aircraft remains in place. If there is air in the brake lines you should feel the pedal drop or require ‘pumping’ to hold the airplane in place. Using the parking brake is discouraged because if not set properly it is not uncommon to look up only to find the airplane is in the taxiway and on its way to the runway or the edge of the taxiway.
Magneto Checks: This is not just a check of the Left or Right Mag as indicated on the switch, but it is a check of the engine’s entire electrical path – The magneto operation, the ‘P’ lead connection, the plug wiring, the spark plugs and the plug electrodes. An RPM drop of 200 RPM or more is typically indicative of a fouled spark plug (improper leaning during taxi – even at sea level), a mistimed magneto, a failed spark plug or broken plug wire. If the manufacturer’s fouled plug cleaning process doesn’t work to bring it back to a normal RPM drop during the mag check, it is time to taxi the airplane back to the shop.
Propeller Governor Control: Airplanes with an adjustable pitch prop will need to cycle the control usually two to three times to ensure oil is moving through the governor and hub. Usually this is performed quickly and there should be a noticeable change in propeller speed and a small dip, then recovery, in oil pressure. Those airplanes with feathering props will typically have a different RPM setting and drop limit to check the feathering function.
Vacuum/Suction: For those airplanes still using vacuum-driven gyro instruments it is important to ensure that sufficient vacuum exists to keep those instruments stable. It is also a good time to know if the airplane has a redundant vacuum pump or back-up electric instruments should there be a failure.
Panel Lights Check: Newer aircraft will have a test function for their panel or instrument lights to indicate an abnormal condition. This is normally checked with the engine at high RPMs since, at idle, it is not uncommon to see a ‘Low Vacuum’ or ‘Low Voltage’ alert. At some point during the instrument checks there should be a verification that the generator/alternator is producing current and there should be a positive indication on the ammeter.
Idle Check with Carburetor Heat: Bringing the engine all the way to the throttle’s idle stop is a way to ensure the proper idle mixture setting has been set by the mechanics. This is important so that during a landing the engine has enough fuel to continue running in case a go-around is needed. For those carbureted engines pulling carb heat on at idle should bring the RPMs low, but again it should not kill the engine. If the engine does die with carb heat on, it could indicate an exhaust leak which can be very problematic.
These checks are a deliberate stress test of sorts to ensure the engine is warmed up and major components are observed for possible, obvious or significant failures. This is especially important if it is the first flight of the day or if the airplane has been sitting for days. The goal is to catch anything wrong while still on the ground, rather than taking a problem with you in the air.
The airplane’s AFM/POH is going to be the final authority as to which checks should be performed, in which order, and what limits need to be adhered to, but hopefully this explains a little better what is going on behind each step.