When Should You Start A 'Pilot's-Discretion' Descent?

When ATC gives you a "pilot's discretion" descent under IFR, how do you plan for it?

What Is A "Pilot's Discretion" Descent?

According to the FAA, when ATC issues a clearance to descend at pilot's discretion (PD), pilots may begin the descent whenever they choose, and at any rate of their choosing.

Pilots are also authorized to level off, temporarily, at any intermediate altitude during the descent. However, once the aircraft leaves an altitude, it may not return to that altitude.

This allows you a lot of flexibility in timing a descent for fuel burn requirements, weather considerations, efficiency, etc.

You might also hear PD clearances included in segmented descents, and the words "pilot's discretion" may not be included. Let's say ATC tells you to "cross the Belair VOR at or above 11,000 feet, descend and maintain 6,000 feet." You can begin your descent at any time and point, as long as you can cross the Belair VOR at 11,000 feet. After meeting this crossing restriction, you are expected to descend at a normal rate until reaching 6,000 feet.

The Argument FOR Descending Right Away

By the time ATC issues you a pilot's discretion clearance, you're probably approaching the point where a normal rate descent would occur anyway. If you start the descent right away, even a shallow one, there's no chance you'll forget to start descending. It's just one less thing to worry about while you're busy planning an arrival.

The Argument AGAINST Descending Right Away

Unless a substantial descent rate is required, you should delay your pilot's discretion descent for maximum efficiency. Staying high, you'll burn less fuel (in both normally aspirated pistons, as well as jets) and you'll fly at a higher true airspeed.

Another upside for jets is that when a faster descent rate is required for the descent, they require little thrust, or even flight idle.

You Can Request "Pilot's Discretion"

Did you know that you can request a PD clearance? Let's say you're flying over an overcast layer of clouds that have icing conditions. You could request a pilot's discretion descent to pick your way through layers of clouds if you want to avoid ice. This type of descent can also help to minimize the time spent in turbulence, by allowing you to level off at an altitude where the air is smoother.

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Announcing That You're Vacating An Altitude

Now that we've covered how you get a PD clearance, there's one last thing to cover: vacating an altitude. According to the AIM paragraph 5-3-3, you should make a report to ATC "When vacating any previously assigned altitude or flight level for a newly assigned altitude or flight level."

There's a fair amount of debate among pilots and controllers whether you should report leaving an altitude on a PD clearance, but according to the AIM, you should. And as long as the frequency isn't jammed up with controllers giving clearances, it's typically better to over-inform, than to under-inform, when you're talking to ATC.

Some Companies Have Specific Policies

Some airlines have policies in place which require their pilots to descend immediately when given a "pilot's discretion" clearance within 5,000 feet of their current altitude. This prevents them from forgetting to descend and busting a crossing restriction. At the same time, it allows some flexibility to delay more substantial descents for fuel savings and overall efficiency.

Even if you don't fly for an airline, try setting similar standards for yourself to avoid missing a clearance or crossing restriction.

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