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There are a few times when you should use the phrase "unable" with ATC...

1) You're going too fast to safely exit the runway at the taxiway the tower has directed you towards.

2) You can't accept a "land and hold-short" (LAHSO) clearance due to landing performance.

3) You've been given a hold in the middle of icing conditions.

4) The runway in use exceeds your aircraft's wind limits, and you need a runway change.

5) You're unable to follow an instrument procedure due to equipment limitations, and you need an alternate procedure.

6) You've been cleared late for a last-minute descent, and you're not able to comfortably make a crossing restriction.

7) You're unable to maintain a 500 foot-per-minute climb rate under IFR.

8) ATC gives you a speed restriction that you can't maintain.

9) You've been assigned an altitude that you're not comfortable or capable of flying at because of weather, oxygen requirements, etc.

10) In short...any time you're given a clearance or instruction that affects safety of flight or exceeds your aircraft's limitations.

Keep in mind, avoid giving a simple "unable" response without closing the loop. If time allows, briefly explain why you're unable to comply with an instruction, and suggest an alternative

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