Unreliable speed explanation

Unreliable speed explanation

Unreliable speed 

On this post I am going to talk about unreliable speed.

First of all I am going to explain what an air data reference (ADR) is.

The Air data reference system (ADIRU) is a part of the navigation system and has two parts air data reference (ADR) and inercial reference (IR).

ADR : recieve information of probes, pitot tubes and static ports. Which means information about speed, altitude, angle of attack.

IR: recieves information about from GPS. Which means notification points, airways …..

Why does an unreliable speed occur ?

The most probable reason it is an obstruction of the pitot tube or static ports. In the following video it is shown how a blockage affects us:


Event occurred on 2008, A330 of  Air France

Let´s explain how to react against this situation in an Airbus 320. 

The captain probes feed the ADR1  and the captain primary flight display. The ADR2 is fed by the first officer probes, and gives information to the F/O primary flight display, and the standby probes give information to the standby instruments.

This system works as follows : when the data provided by one source diverges from the average value, the systems automatically reject this source and continue to operate normally using the remaining two sources. However we could have the situation that the average is wrong , which leads on a unreliable speed undetected by th  ECAM.

Possibilities :

  • Normal situation


  • One ADR fault :   

   This situation does not have an effect, as the system refuse the failed ADR.

  • Two or three ADR fault :

     This situation it is the most problematic as the system gives us an erroneous information.

How to deal with it?

1.- It is very important, and the same time difficult to recognize this situation. The only way to recognize it, is to have an instrument cross check.

2.- apply the procedure


As you can see in the procedure there are “two parts” , one when safety in compromized and the rest cases.

When safety is compromised, we have to apply the memory item , which basically consist on disconnecting all the automatisms, a certain pith and power setting, gear up , configuration flap 3 maximum and speed brakes retracted . This is not only when we are taking off , or close to the ground, also flying at FL370. Imagine that something wrong at FL370 is going on, so first you have to put the power setting to climb , and the pitch to 5º, if we do so we are on the safe zone for sure.

Then when we have have the situation “undercontrolled” we have to apply the rest of the procedure.



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