QRH: avoid icing conditions
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QRH: avoid icing conditions
Hi,
I was reading 737 QRH (online document) and I found this statement "Avoid icing conditions" for certain cases such wing-body overheat or bleed trip off.
First I want to say I am not pilot and I don't know if this document is updated. If I am not mistaken icing conditions mean: TAT below 10C or visible moisture. Here are my questions:
a) How you can avoid icing conditions? Can you predict icing conditions? Can the weather radar give you a hint about icing conditions before you enter a zone with icing conditions?
b) What you will do if you can't avoid icing conditions (with wing anti ice valve closed or no bleed air)?
c) What are the conditions of dispatching an aircraft if the wing anti ice valve is inoperative?
Feedback appreciated.
I was reading 737 QRH (online document) and I found this statement "Avoid icing conditions" for certain cases such wing-body overheat or bleed trip off.
First I want to say I am not pilot and I don't know if this document is updated. If I am not mistaken icing conditions mean: TAT below 10C or visible moisture. Here are my questions:
a) How you can avoid icing conditions? Can you predict icing conditions? Can the weather radar give you a hint about icing conditions before you enter a zone with icing conditions?
b) What you will do if you can't avoid icing conditions (with wing anti ice valve closed or no bleed air)?
c) What are the conditions of dispatching an aircraft if the wing anti ice valve is inoperative?
Feedback appreciated.
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QRH: avoid icing conditions
...which means at nighttime, if you can see ground lights or stars and your lights aren't illuminating a cloud, you're also essentially ok.
Join Date: Jul 2001
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No the weather radar won't help you (much). Obviously you can use the radar to avoid flying through precipitation.
Otherwise you have to use common sense:
E.g. look out the window. Even at night you will see when you are flying in clouds. As well you can keep an eye on outside air temperature. Engine anti-ice is still available, and the wings are not going to accumulate ice at e.g. -30 deg C.
Risk of wing icing is predominantly from -10 deg to a few + deg C. And high moisture contents, which can be seen early on as small droplets forming under the wipers.
A way to deal with it is not to stay in the layer with those temperatures for too long, like descending faster through it, to minimise exposure. Another way is simply to increase speed which will will increase the total air temperature.
Otherwise you have to use common sense:
E.g. look out the window. Even at night you will see when you are flying in clouds. As well you can keep an eye on outside air temperature. Engine anti-ice is still available, and the wings are not going to accumulate ice at e.g. -30 deg C.
Risk of wing icing is predominantly from -10 deg to a few + deg C. And high moisture contents, which can be seen early on as small droplets forming under the wipers.
A way to deal with it is not to stay in the layer with those temperatures for too long, like descending faster through it, to minimise exposure. Another way is simply to increase speed which will will increase the total air temperature.
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A) Avoid icing conditions by avoiding any cloud when TAT temperatures are <10C, easily done in daytime by circumnavigation, at night this may be difficult to achieve, use of fixed landing lights is an option to see a little bit ahead of you. The weather radar only is able to pick up large masses/density of moisture, thus unsuitable to detect clouds unless they contain eg rain etc.
B) If you cannot avoid icing condition you have to make a risk assessment, what is safer to do. Remain aloft or descent through a layer or possibly even land within icing conditions. All depends on fuel remaining, alternates available, weather conditions aloft and on ground... Take the lesser of the evils presented to you.
C) MEL refers that dispatch is allowed with Wing Anti Ice Valve inoperative provided:
Dispatching with an engine and nose cowl anti-ice valve inoperative open will cause the associated COWL VALVE OPEN light to illuminate bright and the associated thermal anti-ice (TAI) indication to also illuminate amber. Prepare the airplane for flight with wing anti-ice valve inoperative.
B) If you cannot avoid icing condition you have to make a risk assessment, what is safer to do. Remain aloft or descent through a layer or possibly even land within icing conditions. All depends on fuel remaining, alternates available, weather conditions aloft and on ground... Take the lesser of the evils presented to you.
C) MEL refers that dispatch is allowed with Wing Anti Ice Valve inoperative provided:
Dispatching with an engine and nose cowl anti-ice valve inoperative open will cause the associated COWL VALVE OPEN light to illuminate bright and the associated thermal anti-ice (TAI) indication to also illuminate amber. Prepare the airplane for flight with wing anti-ice valve inoperative.
- (Both inop) both are secured closed and provided the airplane is not operated in known or forecast icing conditions
- (One inop) may be inoperative open provided:
- (Except for engine start) associated manifold is depressurised when outside temperature is above 10C
- Associated engine bleed thrust limits are followed when the manifold is pressurised
- Air conditioning and pressurization requirements are followed when one manifold is depressurised.
- When the left Wing Anti-ice Valve is failed open, do not use APU bleed air for air conditioning purposes on the ground. APU may only be used for engine start.
- When the right Wing Anti-ice Valve is failed open, APU bleed air may be used for air conditioning purposes on the ground using only the left AC pack and with the isolation valve closed.
- The appropriate engine bleed thrust limits to be used with the valve failed open are the normal limits for wing anti-ice ON.
- The isolation valve switch must remain in the CLOSE position until the associated manifold can be pressurized by turning the engine bleed on
- When dispatching with a single engine bleed on for takeoff due to one wing anti-ice valve failed open, (airplane pressurized), VMCG should be determined based on AC packs OFF. Takeoff performance should be based on AC packs in ON/AUTO.
- Increase trip fuel burn by 2.8%.
- To ensure sufficient air for engine start, only one valve may be inoperative open (due location of anti ice valve relative to engine exhaust area).
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Avoid icing conditions by avoiding any cloud when TAT temperatures are <10C
Not really necessary if you have high negative temperatures or more than +2-3 deg C. The "less than 10 deg C" icing criteria applies to the engines due to the venturi effect of the cowling and high speed of the air passing through. You are not going to get ice buildup on the leading edges at high negative temperatures or more that +2-4 degs C.
Hence you only have to avoid clouds and temp range of -10 to +5 deg C. If you have those conditions during approach = divert. Or ask for pilot reports from aircrafts ahead of you, if available.
If you have those conditions (unavoidable) during descent, inform ATC about your problem and make sure to get a clearance that will let you pass the layer unrestricted (no level offs and and free speed). Personally I would reduce speed to minimum clean and wait for the Total Air Temperature to hit -20 to -15ish, then I would increase to max speed as possible according to turbulence. A 130 knots speed increase should easily give you a 20 deg bump in the Total Air Temperature and a subsequent VERY high rate of descent to minimize the exposure through the layer (use vertical speed of -6000 to accelerate faster than using level change).
No need to panic. "Remain aloft" is not really a permanent solution either.
I flew the A-10. It didn't have anti-ice of any kind. I flew it in the Northeast part of the US. In the winter. In cloud. We'd have ice forming on the gun, the wings, the bombs for God's sake. Anti-ice saves compressor blades (yes. We damaged them) and makes pilots feel better. Beyond that, I'm not so sure. I also flew the C-5 without wing anti-ice for hours in the ILS pattern, in winter, in clouds, never saw much wing icing. Big wing, not so much of a problem.
