When can you descend from the enroute altitude?

When ATC issues a clearance or instruction, pilots are expected to execute its provisions upon receipt. In some cases, ATC includes words that modify their expectation. For example, the word “immediately” in a clearance or instruction is used to impress urgency to avoid an imminent situation, and expeditious compliance is expected and necessary for safety. The addition of a climb point or time restriction, for example, does not authorize pilots to deviate from the route of flight or any other provision of the ATC clearance. If the pilot receives the term “climb at pilot’s discretion” in the altitude information of an ATC clearance, it means that the pilot has the option to start a climb when they desire and are authorized to climb at any rate, and to temporarily level off at any intermediate altitude as desired, although once you vacate an altitude, you may not return to that altitude. When ATC has not used the term nor imposed any climb restrictions, pilots should climb promptly on acknowledgment of the clearance. Climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet below the assigned altitude, and then attempt to climb at a rate of between 500 and 1,500 fpm until the assigned altitude is reached. If at any time the pilot is unable to climb at a rate of at least 500 fpm, advise ATC. If it is necessary to level off at an intermediate altitude during climb, advise ATC.

When ATC issues the instruction, “Expedite climb,” this normally indicates that the pilot should use the approximate best rate of climb without an exceptional change in aircraft handling characteristics. Normally controllers inform pilots of the reason for an instruction to expedite. If flying a turbojet aircraft equipped with afterburner engines, such as a military aircraft, pilots should advise ATC prior to takeoff if intending to use afterburning during the climb to the en route altitude. Often, the controller may be able to plan traffic to accommodate a high performance climb and allow the pilot to climb to the planned altitude without “expedite” clearance from restriction. If you receive an ATC instruction, and your altitude to maintain is subsequently changed or restated without an expedite instruction, the expedite instruction is canceled.

During en route climb, as in any other phase of flight, it is essential that you clearly communicate with ATC regarding clearances. In the following example, a flight crew experienced an apparent clearance readback/hearback error, that resulted in confusion about the clearance and, ultimately, to inadequate separation from another aircraft. “Departing IFR, clearance was to maintain 5,000 feet, expect 12,000 in 10 minutes.” After handoff to Center, the pilot understood and read back, “Leaving 5,000 turn left heading 240° for vector on course.” The pilot turned to the assigned heading climbing through 5,000 feet. At 5,300 feet, Center advised assigned altitude was 5,000 feet. The pilot immediately descended to 5,000. Center then informed the pilot that there was traffic at 12 o’clock and a mile at 6,000. After passing traffic, a higher altitude was assigned and climb resumed. The pilot then believed the clearance was probably "reaching" 5,000, etc. Even the readback to the controller with "leaving" did not catch the different wording. “Reaching” and “leaving” are commonly used ATC terms having different usages. They may be used in clearances involving climbs, descents, turns, or speed changes. In the flight deck, the words “reaching” and “leaving” sound much alike.

For altitude awareness during climb, pilots often call out altitudes on the flight deck. The pilot monitoring may call 2,000 and 1,000 feet prior to reaching an assigned altitude. The callout may be, “two” climbing through the transit to go altitude (QNH), both pilots set their altimeters to 29.92 inches of mercury and announce “2992 inches” (or ‘standard,’ on some aircraft) and the flight level passing. For example, “2992 inches” (standard), flight level one eight zero. The second officer on three pilot crews may ensure that both pilots have inserted the proper altimeter setting. On international flights, pilots must be prepared to differentiate, if necessary, between barometric pressure equivalents with inches of mercury, and millibars or hectopascals, to eliminate any potential for error. For example, 996 millibars erroneously being set as 2996.

For a typical IFR flight, the majority of in-flight time often is flown in level flight at cruising altitude from top of climb (TOC) to top of descent (TOD). Generally, TOD is used in airplanes with a FMS and represents the point at which descent is first initiated from cruise altitude. FMS also assist in level flight by cruising at the most fuel saving speed, providing continuing guidance along the flight plan route including great circle direct routes, and continuous evaluation and prediction of fuel consumption along with changing clearance data.

During the en route descent phase of flight, an additional benefit a FMS is that it provides fuel saving idle thrust descent to your destination airport. This allows an uninterrupted profile descent from level cruising altitude to an appropriate MIA, except where level flight is required for speed adjustment. Controllers anticipate and plan that the pilot may level off at 10,000 feet MSL on descent to comply with the 14 CFR Part 91 indicated airspeed limit of 250 knots. Leveling off at any other time on descent may seriously affect air traffic handling by ATC. It is imperative that pilots make every effort to fulfill ATC expected actions on descent to aid in safely handling and expediting air traffic.

ATC issues speed adjustments if the flight is being radar controlled to achieve or maintain required or desired spacing. They express speed adjustments in terms of knots based on indicated airspeed in 10 knot increments except that at or above FL 240 speeds may be expressed in terms of Mach numbers in 0.01 increments. The use of Mach numbers by ATC is restricted to turbojets. If complying with speed adjustments, pilots are expected to maintain that speed within plus or minus 10 knots or 0.02 Mach.

Speed and altitude restrictions in clearances are subject to misinterpretation, as evidenced in this case where a corporate flight crew treated instructions in a published procedure as a clearance. The aircraft was at FL 310 and had already programmed the ‘expect-crossing altitude’ of 17,000 feet at the VOR. When the altitude alerter sounded, the pilot advised Center that we were leaving FL 310. ATC acknowledged with a “Roger.” At FL 270, Center questioned the pilot about the aircrafts descent. The pilot told the controller that the reason for the descent was to cross the VOR at 17,000 feet. ATC advised the pilot that he did not have clearance to descend. What the pilot thought was a clearance was in fact an “expect”clearance. Whenever pilots are in doubt about a clearance it is imperative they request clarity from ATC. Also, the term “Roger” only means that ATC received the transmission, not that they understood the transmission. “Expect” altitudes are published for planning purposes and are not considered crossing restrictions until verbally issued by ATC.

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.

Fuel Efficiency

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.)

Before issuing a pilot’s discretion descent, ATC has to ensure an enormous block of vertical and horizontal airspace will remain clear of traffic conflicts.

Your Role

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.”

Change Happens

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.

Pet Peeve

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.