B737 Emergency Descent & Anti-Ice

[ImbracableCrunk]

My company's B737 QRH contains a note in the emergncy descent items that says go to idle, except to increase thrust as necessary for anti-ice operation.

I seem to recall that there was an issue with early CFM-56 EECs or PMCs that required a manual increase in engine speed to maintain icing protection, but that it had been fixed. This, I thought, was evidenced by the increase in N1 when the engine anti-ice is on.

The Question: Is the QRH note just a vestigial note that have been removed, or is there something more?

[STBYRUD]

As you correctly said, the EECs compensate by setting the idle N1 to somewhere around 40% if I remember correctly when TAI is used - I guess its in there maybe to cover the unlikely case that you had a reversion to alternate mode before your emergency descent?

[ImbracableCrunk]

I suppose a protection against a manual reversion is possible. How does one know if there is enough N1 for anti-icing? I've flown planes that had a "Low N1" light or some such caution, but I haven't found a min N1 yet for the 737. Time to unearth the FCOM.

[STBYRUD]

Well, since the TAI valve is pressure actuated you might get a bright COWL VALVE OPEN light if the pressure is insufficient to keep it open...

[BOAC]

I'm pretty sure 40 is good. STBYRUD - it is not the valve activation that is in question - it is the supply of deicing air.

[lomapaseo]

I also believe it is to have enough warm air available to protect the aircraft from ice. The engines are not a concern for a single limited encounter (the time it takes to descend)

[de facto]

If I remember correctly the 45 % minimum N1 during idle thrust and anti ice selected is for older 737 classic series.
Normally it is placarded to remind the crews the same way as when you have aircrafts with center tank pumps limitation.

If you select anti ice ON while the thrust is idle without keeping 45%N1 you will get a LOW IDLE light above the engine primary display(somehwere in the middle:\ )

[ImbracableCrunk]

If you select anti ice ON while the thrust is idle without keeping 45%N1 you will get a LOW IDLE light above the engine primary display(somehwere in the middlehttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/wibble.gif )


That light doesn't exist in the NGs - which is the only variant I'm in right now.

Does anyone know where to find a reference for the min N1 for icing? I think it was edited out when they updated the engine controls, but I'm sure someone knows where to find it.

[aterpster]

Unless some fundamental aviation concepts have changed, engines must use anti-ice; wings use de-ice.

If an emergency descent is being made into moist conditions conducive to rapid ice accumulation judgment hopefully comes into play.

It's like a "canned" certification emergency descent done in a P.C. to 10,000 feet. That doesn't work very well over the Rockies or the Alps.

[lomapaseo]

Unless some fundamental aviation concepts have changed, engines must use anti-ice; wings use de-ice.


Depends on when you turn it on as well as the heat load on the ice.

In limiting conditions the ice forms and sheds anyway. The fundamental concept is to limit the amount that builds on a surface to something that won't cause problems in-flight.

[BOAC]

Depends on when you turn it on - IF you 'turn it on' IAW your ops manual (or at least all the ones I know of) you will never 'anti-ice' the wings! Strictly not advised.

[de facto]

Crunk,

Neither do PMC exist on NGs.

There is no mention of minimum N1 during anti ice selection but a minimum for
ground idle (58%)and flight idle(59%).
When you select anti ice N1 and N2 will increase(72-79%).
EEC will take care of it, so dont you worry:E

[reivilo]

- IF you 'turn it on' IAW your ops manual (or at least all the ones I know of) you will never 'anti-ice' the wings! Strictly not advised.Actually it is written in the OPS manual....
"Use wing anti-ice during all ground operations between engine start and takeoff when icing conditions exist or are anticipated, unles the airplane is, or will be protected by the application of Type II or Type IV fluid".

And further on:

"In flight, the wing anti-ice system may be used as a de-icer or as an anti-icer. The primary method is to use it as a de-icer by allowing ice to accumulate before turning the wing anti-ice on. This procedure provides the cleanest airfoil surface, the least possible runback and the least thrust and fuel penalty.
The secondary method is to use wing anti-ice before ice accumulation. Operate the wing anti-ice system as an anti-icer only during extended operations in moderator or severe icing conditions, such as holding."

Source: Boeing 737 FCOM, SP 16, Adverse Weather

[BOAC]

Quite right, rev, but para 2 discouraged except in those conditions specified since you can finish up with water (or melted ice) constantly refreezing over the upper surface and you can do nothing about it. You are right - I should not have said 'never'! (Repeat after me - never say never - never say never....)

I have always preferred (and been taught) to wait until I have an accumulation and then de-ice.

[reivilo]

I have always preferred (and been taught) to wait until I have an accumulation and then de-ice.
So have/been I ;)

[lomapaseo]

have always preferred (and been taught) to wait until I have an accumulation and then de-ice.

Two things going for you using this procedure

The ice accumulation acts as an insulator between the (warm) anti-ice air and the much colder outside air. Thus it takes less heat to raise the interface temp between the ice and metal (sic) it's adhering to, allowing it to shed.

The accumulation of ice itself results in increased drag (hopefully not too much affect on operating speed) and thus increased windage loads on the ice helping it to dislodge

[ImbracableCrunk]

When you select anti ice N1 and N2 will increase(72-79%).
EEC will take care of it, so dont you worryhttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/evil.gif

That's my point. Why put the note in if the EECs will do it? Old school PMCs maybe didn't, but now they and EECs do.

[aterpster]

Imbraceable Chunk:

That's my point. Why put the note in if the EECs will do it? Old school PMCs maybe didn't, but now they and EECs do.

I flew only one airplane where the EECs would do that. But, only if the engine anti-ice was turned on.:)

[A37575]

There is no mention of minimum N1 during anti ice selection but a minimum for
ground idle (58%)and flight idle(59%)

You are sure going to be taxiing like the clappers and hot dragging brakes if you use minimum 50 percent N1. And flight idle of 59 percent N1 is unheard of in a CFM 56. Are you sure your figures are for a 737?

[de facto]

Typooooo!
I meant N2:suspect:

did your airline ever fly classics before you joined?maybe the note you mentioned is just due to your company not updating their sop
?