STRANGE - From the ASB CEB–A–72–6048 issued on 2nd OCTOBER (and revised on 6th) and further covered by Bell ASB 407-03-61 (issued 21nd NOVEMBER):
"Recently, an incident was experienced in which a 4th–stage turbine wheel (23066744) was found to have an over blended, thin trailing edge. Although blending is a common procedure for turbine wheels, in this particular case the turbine wheel blade trailing edge was blended beyond print requirements. All Rolls-Royce Model 250–C30, –C40 and –C47 series 4th–stage turbine wheels (23066744), S/N X504345 and earlier require a one-time inspection to verify that the blades do not have a thin trailing edge."
"For 4th–stage turbine wheels (23066744, S/N HX76702 to HX76755): Compliance Code 2. Turbine wheels which have over 1000 hours must be inspected within the next 150 hours. Turbine wheels which have less than 1000 hours must be inspected within the next 300 hours or at 1000 hours, whichever comes first."
Inspection is with a simple go-no gauge and can be done installed. Failure requires removal of wheel and fitment of new blades.
NOW.... OPTIONS
a) this ASB could be just a coincidence and there is a further problem with 3rd stage (though no ASB or FAA EAD issued as yet to my knowledge)
or
b) 10 weeks after the ASB is issued, an operator with many affected 407s with >1000 hours on their turbine wheels have used up the 150 hours and not having yet done the checks are thus grounded
or
c) 10 weeks on the inspection failure rate is outstripping the overhaul shops' ability to replace the blades
or
d) things have moved on, the problem is (or is seen as) more serious and PHI have judged the inspection is not sufficient