When running through the checklist and after turning the master switch on, you are required to verify that three warning lights are on – VAC, ALT and OIL.
But why do these light up at this point and why check them?
These three lights are battery powered. Hence why they light up when you turn the ignition key before starting the engine. These lights should then go out when the engine starts.
The alternator light shows when a current is not coming from the alternator, meaning the battery is not being charged and will fail.
In UK registered aircraft, a second, brighter red light is also attached to bring more notice to the ALT failure light as it has been noted that pilots can miss the orange warning light on bright days.
The vac and oil lamp are pressure sensitive and therefore light up when the engine is not turning.
The oil warning light indicates low oil pressure. If this illuminates in flight it indicates an issue with the oil flow to the engine and can become serious. Below 35psi and the engine needs to be shutdown immediately.
When in flight if you see one of these warnings lights on it is important to note and take action.