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.
- (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.
To achieve the MEL relief above:
- 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).