Airplanes, like all technical gadgets, face bugs, faults and defects from time to time. When preparing for a flight, the crew is notified of any defects on the airplane and the defects are compared to the so-called minimum equipment list which includes all the devices and equipment that must be functional. For example, of the three VHF radios used to communicate with air traffic control, the so-called “radio number two” (VHF-2) can be out of order in our Airbus planes for three days provided that the VHF-1 and VHF-3 are fully functional (usually just one radio is used at a time).
Some defects require technical and/or operational procedures, and the captain is responsible for overseeing that these procedures are carried out. The list is quite conservative. For example, if the light of the emergency exit in the cabin is not working, only as many passengers will be allowed on the aircraft as would be approved by authorities when the whole emergency exit is out of order. Even though, in case of an emergency, the door would function just as it should.
Naturally, a defect can appear during flight. In order for the defects not to risk flight safety, every important device has a back-up system and the most vital ones even have back-up systems for the back-up systems.
Here’s an example: landing gear. Landing gear always has two sets of tyres so it won’t be a disaster even if a tyre deflates or there is a problem with a brake. The landing gear also has two detector systems showing the position of the gear. This means that a light or sensor defect will not cause unnecessary emergency preparations in the cabin.
The landing gear also has at least two operating systems. If the gear cannot be lowered with the normal hydraulic system, it can also be dropped down manually. On the other hand, there is only one nose gear steering system as it is not a necessity for flight safety. Landing can be done in an absolutely normal manner even if the nose gear steering system is defective. The only thing is that the plane cannot taxi to the parking spot without assistance. If the way from the runway to the taxiway is a gentle turn (so called high speed exit), the aircraft can be steered away from the runway used by other aircraft using aerodynamic controls (rudder) and asymmetrical braking.
In case something fails when the plane is in the air, the flight crew will go through an electronic or paper check list. With the help of the list, they will try to reset the system. If resetting does not help, the crew will go through the list so that every influential matter will be taken into account. In sudden situations, the checking of the lists will start only after the “known by heart” items. All these are practised annually in a simulator. Also, the cabin crew go through yearly training regarding what to do in case of an emergency.
One of the challenges of being a pilot when something is not working as it should, is that you cannot just park your plane on a cloud for further investigations and democratic decision-making. Decisions have to be made in a changing environment where the pilot must take into account the type of the defect, weather, sufficiency of fuel, the lengths of the runways at alternate airports, direction of the wind and many other things. Sometimes the passengers must just accept the pilot’s decisions, for example landing in an alternate airport. Of course, when the situation is over, the goal is to get every passenger to their destination with minimum delay.