|Quoting JetMech (Reply 2):|
The actual surface of the fan case adjacent to the tips of the fan blades is not metal. It is an abradable material which is soft enough for the tips of the blade to rub away
The P&W JT9D engine as installed on the 747 classic has an abradable seal around the fan blades, for optimizing the fan blade tip clearance.
When the engines are overhauled new seals are (sometimes) installed. During the first engine starts of new or overhauled engine sometimes the fan became stuck (zero N1 RPM indicating),
Action was/is : counter rotate the fan and try again.
If the abradable seal was worn to much, due turbulence, hard landings and reverse operation (vibration) a performance and/or SFC loss was noticed.
In extreme circumstances (high angle of attack and to much fan tip clearance) this could lead to a fan stall, a very dangerous situation. Suddenly the thrust delivered by the fan was reduced and then back again, resulting in a violent engine swing. Personally I encountered two fan stalls, when climbing out from ATH
(relative low TOW, high angle of attack and in a turn) on both outboard engines of the 747-200 PH
in the early 80's.
Lowering the angle of attack quickly cured the problem, as stated in the AOM (aircraft operation manual). However after landing we ordered an inspection and it was discovered that the fuse pins of engine strut No 4 were out of tolerance.
Both abradable seals of engines 1 & 4 were replaced.
[Edited 2009-10-29 13:30:16]
Operating a twin over the ocean, you're always one engine failure from a total emergency.