Question regarding Stall Margin for IAE engine on A320

[kikwon]

The chief pilot from our airline just returned from a conference by IAE. The big issue surrounding IAE engine in 2018 were incidents of frequent engine surge during idle descent at low altitudes (around 5,000 ft) many of them unrecoverable. I guess pretty dangerous situation to deal with and unfortunately very frequently happening.

Well, they have disseminated advanced copy of new page that will be inserted in the new edition of FCOM as always. AIRBUS is known for their frequent updates of their books, which doesn't bother me. So, they now want us to turn on engine anti-ice below 10,000ft in order to improve stall margin (I'm guessing this means to increase stall margin), especially when the A/C has its packs or bleeds off for any reason (For instance, the MEL may have O procedure that requires the crew to have them off)

I could just procedurally understand this and apply it in real life, that's easy. But I want to get a little deeper technically and scientifically with the help of smart people in aviation because I just can't seem to understand.

As far as I understand, for IAE engines, it pulls hot air bleed from the 7th stage to use as engine anti-ice. So pulling bleed out of its 7th HPC stage at Let's SAY idle open descent phase of the flight going into our destination, how is turning on the engine anti ice going to help increase stall margin so that we won't run into situations of engine surge/stall where we may have to land with only one engine operative.

Please explain it in both laymans term and professional term, your help would be greatly appreciated guys. Thanks.

AJE from KOREA

[iggy]

Hi Aje,



A stall is when the airflow separates from the airfoil, this also applies to the blades of the engine compressor, which are basically airfoils. When the engine is at idle thrust there is the risk of having too much air pressure accumulated at the front stages of the compressor (the engine is not producing thrust so it is not "swallowing" so much air), something that can slowdown the total speed of the airflow that passes through the blades, causing a stall on them. Usually there is a dedicated valve that opens when the engine is at idle regime, to guarantee a minimum speed on the airflow through the blades by giving the air flow an easy way out (the valves usually are at the sides of the engine, not at the end of it, if I recall correctly). The engine anti ice valves also open this idle valve to guarantee the flow of air (this same vale or another dedicated one, depends on the engine), that is why the procedure.

I was told to do this same thing when I flew the MD80 if I was going to encounter turbulence, as a way of protecting the engine from a possible stall (JTD engines in the MD80 didn't come with FADECS).

Hope this clarifies.

[lomapaseo]

Anything u can do to reduce pressure within the N2 helps prevent compressor stall.. Other factors at work behind the scene are blade tip clearance and moveable vane schedule. The last two are there for efficiency and not practical to dick with. So manual opening a bleed at the right location is a crutch

[FE Hoppy]

Pressure at the intake of the compressor is low. Pressure at the exit of the compressor is high. The air would prefer to move from high to low (that would be a surge). The only thing that prevents it is mass flow of air.
At low rpm there is not much mass flow so the engine has a number of systems to ease the airs passage, primarily opening valves at the high pressure end to offload the pressure and varying the angle of the airflow at the low pressure end through inlet guide vanes and variable stator vanes.
At the design stage these were scheduled to keep the air moving and attached to the compressor blades but nothing is perfect at it would appear that the pressure gradient is still too high in this flight regime. Compressor stall is when a single stage of blades become turbulent, this would normally be at the high end, if this happens it can feed back and cause the stage in front to stall etc leading to a full surge (engine literally coughs it's gut's out the front).
By manually opening any bleed valve you are offloading the high end. Some engines will do this anyway, (PW1524 uses the engine anti ice valves to offload the compressor in lots of regimes) and therefore reducing the pressure gradient and increasing mass flow in the front of the compressor.

[Roj approved]

Some very good information here regarding Engine Stalls, thanks to the contributors.

Something else to consider from the FCOM DSC 30-30-10 p1/2

When an engine anti-ice valve is open, the N1 or EPR limit is automatically reduced and, if
necessary, the idle N1 or EPR is automatically increased for both engines in order to provide the
required pressure.

As I understand it, the second part of that description, an increase in EPR is an important part of preventing the stall.

[lomapaseo]

Some very good information here regarding Engine Stalls, thanks to the contributors.

Something else to consider from the FCOM DSC 30-30-10 p1/2

When an engine anti-ice valve is open, the N1 or EPR limit is automatically reduced and, if
necessary, the idle N1 or EPR is automatically increased for both engines in order to provide the
required pressure.

As I understand it, the second part of that description, an increase in EPR is an important part of preventing the stall.

I believe you are talking about a control function. Many variations in control functions for engines over the years and if we get into these this thread may get very complex.

For example. Many controls don't limit EGT since it's a time limited concern and pilots would prefer in emergencies to decide for themselves. But if you add an after design crutch of opening a bleed by pilot command, then the engine efficiency is going to suffer and EGT increase. As long as you are not trying to extract high power, there should be no problem

just a thought

[IFixPlanes]

Ask your Maintenance for a copy of the TFU 71.00.00.118. (TFU = Technical Follow-Up)

The Problem is a combination of engine configuration (SelectTwo™ with HPC >10.000 cycles and EEC software SCN22), at or below 5,000 feet altitude and no bleed.

[kikwon]

clearly understood captain
you know more than any of our captains here at my airline haha
thank you so much