DCVC wrote:
The Norsk machine (referred to here) had a vespel spline adapter failure, which allowed the remaining pump to reverse flow the oil through the windmilling failed pump, in preference to the more restrictive pressure line - like a short circuit, it took the path of least resistance. The pressure dropped to somewhere around 5 psi or slightly less, which was, at that time, a LAND IMMEDIATELY drill. The crew were lucky that a platform was sufficiently close that they were able to land there instead.
That aircraft had the -101 pump. An initial fix was the -102 pump with improved finishing on the splines and an inspection requirement.
The -103 pump was then introduced which had a check valve thereby isolating the two pumps. In the event of a single pump failure, the remaining pump would produce around 17 psi.
However, an aircraft then had a pump failure (associated with the vespel spline adapter) and the effect of not scavenging the input module became apparent through an action known as churning, whereby kinetic heating of the oil took place within the gears. The crew landed onshore.
An earlier event had also taken place where churning had manifested itself (212man's event?) but that was not associated with a pump failure.
A revised emergency procedure was introduced to address this phenomenon, whereby the engine of the affected module is throttled back to reduce the heating effect. the check valves were also removed from the -103 pumps to create the -104 pump. This was to stop the input modules being filled with oil so rapidly - which was overcoming the ability to scavenge by gravity drain. The remaining pressure of 5 psi is still sufficient to lubricate the MGB adequately.
What a complete engineering screw up. Why can't the engineers stay focused?
Reason for 2 oil pumps: - Design Objective, to provide redundancy if one oil pump fails by providing maximum oil pressure to the MRGB from the remaining oil pump, to provide fail operational capability.
Problem: -103 oil pump design (with check valves) during single pump failure, provides 17psi, however allows non-scavenged input module to overfill by exceeding capacity of gravity drains.
Solution: - Improve capacity of input module gravity drains to maintain 17 psi with check valves, (oops) remove check valves from oil pump with -104 redesign and accept 5 psi as adequate protection with single pump failure (what happened to our Design Objective?).
Testing Objectives: - Determine by testing, how long the MRGB will survive with 17 psi oil pressure? Determine by testing, how long the MRGB will survive with 5 psi oil pressure?
Did the Cougar flight crew inadvertently provide some of the needed test results?