Final answer:
If a PACK fails above certain altitude select cowl anti-ice ON. The altitude at which cowl anti-ice must be turned on following a PACK failure is specific to the aircraft and listed in its operational manuals, and it is designed to prevent ice formation on engine cowlings.
Step-by-step explanation:
Aircraft PACKs (Pneumatic Air Conditioning Kits) are crucial for maintaining cabin pressure and temperature. In aviation, especially in commercial airliners, operating procedures are established for systems management including the use of anti-ice systems.
Pilots must engage the cowl anti-ice system when operating in conditions where icing can occur, which is often at temperatures near or below freezing.
However, the specific altitude at which cowl anti-ice must be selected on after a PACK failure is typically determined by the aircraft manufacturer and can be found in the Aircraft Operating Manual or Quick Reference Handbook for that particular aircraft. As such, without specifying the type of aircraft or manufacturer guidelines, the exact altitude cannot be provided.
In general, the use of cowl anti-ice is to prevent ice from forming on the engine cowlings, which could affect the engine performance and air intake.
Implementing the cowl anti-ice system may vary based on aircraft type, but originates from a common principle of ensuring the safety and proper functioning of the aircraft engines under cold weather conditions where ice can form.