Once Typhoons started operating from forward landing grounds in Normandy, it was found that the dust clouds stirred up by propeller wash consisted of over 80 percent of hard, abrasive material which was damaging the Sabre engines. The sleeve valves in particular were subject to excessive wear and it was calculated that engines would last for three take-offs. As a result, a "dome deflector" was designed and manufactured at great speed by Napier, and within a week most Typhoons had been fitted with it. In operational service these mushroom-shaped air filters, which became red hot, had a propensity for being blown off the air intake at high speed whenever a Sabre engine backfired. They were soon replaced by drum-shaped filters designed by the RAE and Vokes. These had "cuckoo clock" doors in front, which swung open with the pressure changes caused by engine backfires. This standardised filter became Typhoon Mod.420.