As you approach your destination, ATC will clear you to begin a descent from your enroute altitude to some lower altitude. Often descent clearances will come in a series of lower altitudes. This series of step-down clearances is issued to allow you to descend without conflicting with other traffic at lower altitudes.
Occasionally, and in some areas more often than not, ATC will allow you to initiate a descent at your own discretion. The point of a discretionary descent is it allows you to remain at your cruising altitude for as long as possible.
When operating at a cruising altitude, your aircraft’s engine or engines tend to burn less fuel per mile than at lower altitudes. Further, and this is key, by allowing you to determine when to descend, ATC gives you time to reach a point where a continuous descent at idle power is possible.
In simplest terms, a calculated point to begin an idle descent should permit you to reach your destination airport’s traffic pattern altitude while burning the least amount of fuel possible. The calculated point to begin an idle descent is called “top of descent” or “idle descent point”. Your flight instructor should have tips for how to perform this calculation.
When issuing a pilot’s discretion descent, ATC may not allow you to descend all the way down to the lowest altitude where radar vectors to final will occur. In those cases, you may receive a pilot’s discretion descent down to some intermediate altitude. Even then, delaying your descent for as long as possible is an advantage.
What Drives a Pilot’s Discretion Descent
Before issuing any clearance for a flight to descend, ATC must look ahead of that’s flight path and make sure it will be clear of traffic conflicts at lower altitudes. For example, if a plane is cruising at 10,000 MSL and ATC wants to clear that plane down to 5,000 MSL, the controller needs to ensure no aircraft will cross that flight’s path at any altitude between 10,000 and 5,000. This requirement is not difficult to analyze when the controller specifies exactly where the plane begins the descent. The descent path can be forecast over a relatively narrow range of distance. Think of that descent path as a downward sloping tunnel of set dimensions.
Now let’s say the controller is willing to give the flight a descent at pilot’s discretion. When issuing this clearance, the controller cannot exactly predict when the pilot will start his descent. After all, the start of the descent is at the pilot’s discretion. Before ATC gives the clearance, the controller must look ahead for traffic conflicts for many more miles than would be required for a “descend now” clearance. The narrow tunnel of clear airspace required for a “descend now” clearance stretches into a enormous rectangle of vertical and horizontal conflict-free airspace for a pilot’s discretion descent. (See the illustration below.)
When issued a clearance to descend at your discretion, you have some obligations. First, read back the clearance to ATC. This is as simple as repeating the clearance to ATC exactly as issued.
ATC: “Skyhawk 30D, at pilot’s discretion descend and maintain 5,000.”
You: “At pilot’s discretion descend and maintain 5,000. Skyhawk 30D.”
Note this is the correct way to read back this clearance. You’ll hear other pilot’s ham it up and try to sound cool by reading back, “PD to 5,000.” Or worse, “PD to 5.” This is slang and is seriously frowned by the people who wrote the Aeronautical Information Manual, the FAA. ATC will accept this readback because controllers are not language police. Just because ATC rolls with sloppiness on the radio doesn’t mean you should adopt slang as your standard method of operation. Stick to the FAA standards for readback.
With the clearance in hand, it’s time to calculate your idle power top of descent. Your calculation will vary depending on your aircraft’s performance, engine limitations, weather conditions at cruise altitude, and winds aloft throughout the range of altitudes in your descent profile. That’s a lot of consideration. Again, talk to your flight instructor for advice.
Once you reach your top-of-descent point, you should report leaving your cruising altitude to ATC. “Skyhawk 9130D is leaving 10,000, descending 5,000.” ATC will usually respond, “Roger.”
As I said, a large block of airspace is required to be free of traffic conflicts for a pilot’s discretion descent. You can imagine there will be circumstances in which unforecast traffic enters the airspace block after the clearance for a pilot’s discretion descent was issued. If circumstances change, ATC will modify your clearance. The controller may:
- Direct you to descend now, even though your previous clearance was for a pilot’s discretion descent. This means the controller has new traffic that would cross your path if you delayed your descent any longer.
- Restrict the bottom altitude of your clearance to a higher altitude. For example, let’s say you were issued a pilot’s discretion descent to 5,000. After starting your descent, you may hear the controller say, “Skyhawk 30D, descend and maintain 7,000.” This means unplanned traffic is forecast to cross your path below 7,000, making the original clearance to 5,000 invalid.
One Other Variation
In some circumstances, a controller may issue a pilot’s discretion descent in conjunction with a “descend now” clearance. For example, “Skyhawk 30D, descend now and maintain 8,000. Then at pilot’s discretion, descend and maintain 5,000.” This variation means ATC will have traffic crossing your path somewhere between 10,000 and 9,000. By directing you down to 8,000 immediately, he can keep you clear of the known traffic. Once you are level at 8,000, he will have the open airspace to let you continue down to 5,000 at your own discretion.
Air traffic controllers know most pilot’s try to conserve fuel when and where possible. They know most prefer to remain at cruise altitude for as long as possible. With this in mind, a controller will issue a pilot’s discretion descent when possible. If you are told to descend without the option for pilot’s discretion, it means the controller needs you to quickly vacate an altitude or a range of altitudes with potential traffic conflicts.
Some pilots try to game the system by asking the controller if he can change a descent clearance to a pilot’s discretion descent. That’s the same as asking a controller if he ignore the traffic conflict that required the descend now clearance. I can attest, after asking my controller contacts about this, requesting a change to a pilot’s discretion descent is a sure way to make a controller angry.
Pilot’s discretion descents are a good deal. They allow you to remain at cruise altitude for as long as possible, saving fuel. ATC will issue the clearance when and where possible. Take advantage of pilot’s discretion descents when offered, but if not offered, as always, follow the clearance that’s issued.