A320 Family Inadvertent actions on cockpit overhead panel
What happened?
On A320, cases have been reported where ADIRS rotary selector was inadvertently turned OFF during flight, by contact with the CAPT sun visor when deployed or stowed.
The subsequent incorrect use of ADIRS controls can then lead to loss of AP and in some cases flight envelope protections.
Let’s analyse the system
ADIRS rotary selectors are normally “protected” by a “pull to turn” guard.
If “forced” to turn without first pulling the selector switch, this protection guard on the selector can break.
Damage to the “pull to turn” guards removes a layer of protection from inadvertent change of ADIRS mode.
What can we do about it?
When stowing or deploying the sun visor, be carefulnot to hit the rotary switch.
If the “pull to turn” protection is no longer present, report it to maintenance for corrective action.
A quick reminder on ADIRS Overhead Control
ADIRS rotary selectors from CDU (Control & Display Unit) or MSU (Mode Selector Unit) control panels have 3 positions:
– NAV: normal position
– ATT: back-up mode (only attitude, accelerations and heading information)
– OFF: Complete ADIRU shut down (including ADR & IR)
What could be the risks?
In case of ATT or OFF inadvertent activation:
– IR NAV data is not recovered until a new alignment on ground. Change from NAV mode to ATT or OFF mode is irreversible in flight. These positions should not be used in flight unless requested by procedure.
– a layer of redundancy of aircraft flight protection is removed.
In addition, when combined with either:
– Incorrect application of the procedures
– Drift of another IRS during flight
– Another ADIRU under MMEL
depending on aircraft configuration, inadvertent switching may lead to:
– Reversion to Alternate Law
– Loss of AP/FD and ATHR
– Loss of position on ND
Recommendation
In the event of an ADIRU inadvertently switched on OFF or ATT during a flight, follow the sequence of ECAM and QRH checklists with active crosscheck, to avoid any out-of-sequence switching during the recovery procedures.
To sum up
– Be aware of possible interference of the ADIRS selectors when moving the CAPT sun visor.
– ADIRS selectors are “pull to turn” switches – if the mechanism is broken, it should be reported.
– ATT and OFF modes of the rotary selector are not reversible
– Inadvertent actions on ADIRS selectors can remove aircraft protection layers.
– Crosscheck any movement of the ADIRS selector.
Find out more!
If you want some more information about this subject, you can refer to:
– Related FCOM Procedures:
– NAV IR 1(2)(3) FAULT
– NAV ADR 1(2)(3) FAULT
– [QRH] IR Alignement in ATT Mode
– D10X poster on Inadvertent cockpit actions on overhead panel