Cockpit airspeed indicator

What happens if? – The airspeed indicator fails

Tim Takeoff
24.11.2020
3 minutes

An old aviator’s saying says: speed is life, and life is speed! This is one of the most elementary things in flying: speed.

In order for rigid flying objects to stay above the ground, a medium like our air must flow around them. Due to favourable shapes and angles of attack, however, in the end an object only flies if another parameter is added: the flow velocity. We are talking about the air in relation to a body around which air flows, for example the wing of an airplane. In aviation we speak of the “speed”, originating from seafaring.

A very diverse range of flight conditions

Every new aircraft is precisely tested, flown and measured. All speed ranges are documented and optimized for the respective application. All configurations, weights, with landing gear, flaps, in case of engine failure and also at different altitudes are taken into account.

The combination of all these things always results in an approximate airspeed – an absolutely vital insight. If the pilot has no information about the speed, he can still continue to work. How does that work?

What is speed and how does the airspeed indicator work?

How is speed measured and how is it displayed? In a car, the speedometer can convert the number of revolutions of the wheels into speed. In an airplane this does not work, the speed of the airflow must be measured. First of all, the ambient pressure is measured via a static port. Now the total pressure is taken via a so-called pitot tube (pitot probe). This tube is located in the free airstream. Now the static pressure is subtracted from the total pressure and a certain pressure value is obtained, which is assigned to the airspeed in the unit “knots”. 1 knot corresponds to one nautical mile (1.852 kilometres) per hour. Thus the affinity of flying to seafaring is omnipresent.

airspeed indicator - altimeter - horizon
Airspeed indicator (left), attitude indicator (centre), altimeter (right) – Boeing 737-800 (Wikimedia Commons – User Trailer – Public Domain)

If a problem occurs, such as a clogged pitot tube or other pressure drops, the airspeed indicator may give incorrect values or fail completely. However, a commercial airliner has multiple redundancies in this area as well. Since the failure of the airspeed indicator is considered critical, special procedures have been developed to avoid misinterpretations.

Airspeed indicator – Pitch & Power

The manufacturer has developed very exact tables for all important flight conditions, which are essential for certification. They consider all parameters, such as the current weight and the current altitude. If you want to maintain your altitude, climb, sink or take another speed without having the airspeed indicator at your disposal, as a pilot you consult these tables. They tell you very exactly what pitch and power you want to set. This will generate the necessary speed and climb or sink rate. In aviation this is called “flying by pitch & power”. In this way, an airplane can be moved according to these values without any problems and without speed information.

Training in the Simulator & “Memory Items”

In the flight simulator the crews train such events again and again. In case of an emergency, pilots initially assume a safe basic flight attitude from memory. An unreliable airspeed is part of the “memory items”, things that belong to the absolute basic skill set and must be retrievable at any time from memory, without a checklist. Once the aircraft is stabilized, the crew uses the detailed tables and checklists that allow them to bring the aircraft safely to the ground.

by Tim Takeoff

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