The reference All Operators Letter described a problem involving the 2.95 bleed system, whereby one or both bleeds may remain open after engine start-up. A bleed open at high thrust may cause reverser damage or in-flight shutdowns, depend-ing on the aircraft involved. The reference pro-vides a check to be used on the flight deck that may identify that a bleed is open after engine start-up, prior to departure.

Several airlines have experienced difficulty in util-izing the check. The purpose of this letter is to provide guidance to obtain more consistent results from the check. Also, information is provided with respect to the effect of an open 2.95 bleed on en-gine operation.

Airlines have provided detailed, continuous en-gine parameter data during start-up which shows how EGT stabilizes after engine start-up. The ref-erence after start check records EGT data subse-quent to the after start procedure. This requires the engine to be stabilized during start-up in ac-cordance with published start procedures, the af-ter start procedure performed, and the data re-corded. Unfortunately, the reference All Operators Letter did not indicate that time is required subse-quent to the after start procedure and prior to re-cording EGT data, to allow the engines to stabi-lize.

The start data recently provided by the airlines clearly indicates that time is needed subsequent to the after start procedure and before taxi, to allow the EGT of the engines to stabilize. A stable level of EGT is needed prior to checking the difference in EGT between engines for the evaluation to be consistent. Flight crews should be advised that at least a 90 second delay subsequent to the after start procedure is necessary for consistent EGT data.

Pilots should be reminded that there may be times when it is not possible to delay taxi to get stable EGT data. The goal is to do the 2.95 bleed check every flight, but it's better to not do the check than it is to record poor data. If the pilots feel it's im-possible to allow the time to stabilize the engines before recording EGT data, don't do the check on that flight. The check can be done on the next flight, if possible, and compared to the most recent check.

This check is not mandatory, but is provided as an additional tool to identify a possible open 2.95 bleed. We have investigated other ways to check if a 2.95 bleed is open after engine start-up. We have concluded, considering engine performance and flight operational aspects, that the after start check of the reference is the best possibility of a reasonable check.

An open 2.95 bleed will affect engine parameters, however, the pilot may be unaware of bleeds be-ing open during the flight. The airplane operations manual covers all operational possibilities. Typi-cally the engine will operate within limits with a bleed open. If parameters are within limits, thrust is not affected. If parameters do not remain within limits, engine operation is handled with existing abnormal procedures similar to any other engine abnormality.

The effects of an open 2.95 bleed on engine pa-rameters are most noticable at cruise thrust and higher. The actual differences between engines on an aircraft depends on the number of 2.95 bleeds open, and the relative time since overhaul of the engines. During takeoff, climb and cruise, a difference between engines may exist of more than 40 degrees EGT, more than 6% fuel flow, or more than 2% N2.

Corrective action for this problem has been identi-fied. A supply of the parts needed to correct the problem is being actively pursued. As noted in the reference, the after start check is no longer needed after corrective action is taken.