hum, peut être un atterrissage dur, ou un choc
en zieutant divers site qui parle de cette question, depuis pprune,
http://www.pprune.org/tech-log/128138-1 ... swung.html
The full list which we teach is:
a when compass componenets are installed or replaced
b whenever the accuracy of the compass is in doubt
c after a maintenance inspection, if required by the schedule
d after a significant aircraft modification, repair or replacement involving magnetic material
e when carrying unusual ferromagnetic payloads
f when the compass has bee exposed to significant shock (heavy landing?)
g if the aircraft has been struck by lightning!
h after significant modifications to the aircraft radio or electrical system
i if the aircraft has been in long term storage standing on one heading
j if a change to a new theatre of operations involves a large change of magnetic latitude (though quite honestly, I don't think this one has much application in practice - after all they don't re-swing an aircraft compass on every turn-round on long-haul from UK to Singapore and back!)