Communications and ATC Clearance Readback
You are pilot-in-command of a twin-engine turboprop (MTOW 5,700 kg) on an IFR flight from Brunei International (WBSB) to Kota Kinabalu (WBKK). You are at FL180. ATC clears you: 'ABC, climb to FL200, proceed direct to KOTA, maintain 250 knots until advised.' You read back: 'Climb to FL200, direct KOTA, maintain 250 knots, ABC.' ATC then says: 'ABC, correction, maintain FL180, expect FL200 in 10 minutes.' You read back: 'Maintain FL180, expect FL200 in 10 minutes, ABC.' Shortly after, you notice that your altitude is drifting up to FL182. You must decide what to do.
1. What is the correct action and what communication is required?
Read the worked answer
You must immediately correct the altitude deviation and inform ATC. Since you are cleared to maintain FL180, you must not climb above FL180 without a new clearance. The drift to FL182 is a deviation. You should reduce power and descend back to FL180, and inform ATC: 'ABC, correcting altitude, returning to FL180.' ATC may ask for your altitude; you should report 'FL180' once established. If the deviation was caused by turbulence or an autopilot issue, you should advise ATC. The key is to promptly correct and communicate; do not wait for ATC to notice. Also, ensure you do not climb to FL200 until cleared, even if you expect it in 10 minutes.
2. What are the readback requirements for the initial clearance and the correction? Explain the difference.
Read the worked answer
For the initial clearance, you must read back the altitude, route, and speed restriction: 'Climb to FL200, direct KOTA, maintain 250 knots, ABC.' For the correction, you must read back the revised altitude and the expectation: 'Maintain FL180, expect FL200 in 10 minutes, ABC.' The difference is that the initial clearance was an instruction to climb, while the correction changed the instruction to maintain FL180 and expect further clearance. The readback must always include the clearance limit, altitude, and any speed or route changes. The expectation is not a clearance, so you must not climb until you receive an explicit clearance to FL200.