A320 One Pack INOP MEL procedure

[subria023]

Hi all,

I hope some of you could shed some light on this for me. I have a question regarding the the MEL Operational Procedure for one Air Pack Inop. I had an aircraft dispatched with an MEL item of AIR PACK 1 INOP and after going through the MEL I came across the Operation Procedure as follow.

DURING COCKPIT PREPARATION PACK FLOW selector.................................................... ............................................................ .....HI

IN FLIGHT  If BLOWER pb-sw and EXTRACT pb-sw are set to OVRD: ‐ The cabin altitude may reach about 9 700 ft, and ‐ The CAB PR EXCESS CAB ALT alert may be displayed on the EWD

DURING DESCENT  At 10 000 ft, if engine anti-ice (side with PACK pb-sw OFF) is available:
ANTI ICE ENG (side with PACK pb-sw OFF) pb-sw.......................................................... ...ON

My question is, during descent at 10,000 ft, what is the reason to switch on the Engine Anti Ice on the same side of the Inoperative Air Pack? Is this going to be a asymmetrical engine anti ice operation? What is the benefit from doing this? Or I may have misunderstood the procedure completely.

Your input is much appreciated!

Subria

[Dave Therhino]

I'm not familiar with the 320 pack inop procedure, but if it has you operate with the one working pack from the same side engine with the bleed crossfeed closed, they may be having you turn on the anti-ice on the other engine so that its acceleration and deceleration response will be closer to that of the engine supplying the pack, reducing the difference in thrust during any large thrust changes in the approach. Just a guess though.

[Chrome]

That's interesting as we don't have the 'During Descent' part in my outfit's MEL MO.

[The Banjo]

It is to draw more air from the engine to improve stall/surge margins on the engine. A recent software upgrade to improve start temps in hot conditions had the unintended consequence of inducing engine issues at low thrust on descent with no bleed loads. Cannot find the exact info but that is roughly how it is.

[FlightDetent]

subria023 Your understanding seems valid.

Our MEL copy shows the same procedure for IAE engines after the September 2019 revision.

[subria023]

Thank you all for the informative answer. I think your input about maintaining the symmetrical engine performance at low thrust setting does make sense Still no luck finding reference in the FCOM though

[Rostermouse]

No, it is as The Banjo says. It is to improve stall/surge margins on the engine on the failed pack side, because (in simple terms) with less air being extracted from the compressor and a worn engine there is an increased likelihood of stall/surge during descent (higher adverse pressure gradient across the HP comp). One EU airline had 3 within a couple of years. Compare with the procedure for engine stall which asks for the EAI ON to improve margins - same principle, just preventative.

[subria023]

Originally Posted by Rostermouse

No, it is as The Banjo says. It is to improve stall/surge margins on the engine on the failed pack side, because (in simple terms) with less air being extracted from the compressor and a worn engine there is an increased likelihood of stall/surge during descent (higher adverse pressure gradient across the HP comp). One EU airline had 3 within a couple of years. Compare with the procedure for engine stall which for the EAI ON to improve margins - same principle, just preventative.

Rostermouse, thanks again for the clarification. Much appreciated