Helicopter IFR Technique: Communications – Unable and Alternatives 

“Unable” is a standard term in the Pilot/Controller Glossary indicating that a pilot cannot comply with a specific instruction, request, or clearance. 

It should not be used to avoid something that is simply inconvenient. 

Using “unable” for the right reasons is a sign of good airmanship. It demonstrates that you understand the limitations of your aircraft, crew, or operating environment and will not accept a clearance you cannot safely or legally execute. 

Whenever practical, follow “unable” with a concise reason and a feasible alternative. 

A reason helps the controller understand the constraint. An actionable alternative gives the controller another way to maintain traffic flow and separation without first having to ask what your aircraft can do. 

For example, assume you are instructed to hold and given an expected further clearance time that would not threaten the fuel reserves of most fixed-wing aircraft but would be unacceptable for your helicopter. 

You might respond: 

“NXXXAB, unable to hold until that time due fuel. We can hold for fifteen minutes then we will request direct [diversion airport].” 

There are also situations in which you can comply with an instruction but can offer a better alternative. In that case, “unable” is not the appropriate response. Simply make a request. 

Because helicopters represent such a small percentage of IFR traffic, a controller may not immediately consider an option that takes advantage of a helicopter’s unique performance. 

I recently encountered an example while recovering to a non-towered airport within a Class E surface area. 

I was number two for the airport. Because this particular airport has a lengthy instrument approach, the controller told me that I would hold while the first aircraft completed the approach and canceled IFR. I could have accepted the hold, but I recognized that slowing down might provide the required spacing with far less complexity. 

At the time, I was flying at approximately 130 knots. I responded that I could reduce my speed to 60 knots—less than half my current airspeed and a degree of speed flexibility many fixed-wing aircraft cannot match. 

The controller determined that the speed reduction would create the required spacing, asked me to maintain that speed, and moved on to vector another aircraft. 

Slowing eliminated the need for the controller to issue lengthy holding instructions and decreased both of our workloads. 

These examples illustrate an important distinction: 

Use “unable” when you cannot accept an instruction. 

Respond with a suggestion when you can comply but have a better alternative. 

In either case, don’t hesitate to tell ATC what you can do. 

Controllers cannot account for aircraft limitations or performance advantages they do not know about. A concise reason and a workable alternative turn a problem into something the pilot and controller can solve together.