Before the days of nationwide radar coverage, en route aircraft were separated from each other primarily by specific altitude assignments and position reporting procedures. Much of the pilot’s time was devoted to inflight calculations, revising ETAs, and relaying position reports to ATC. Today, pilots and air traffic controllers have far more information and better tools to make inflight computations and, with the expansion of radar, including the use of an en route flight progress strip shown in Figure 3-26, position reports may only be necessary as a backup in case of radar failure or for RNAV random route navigation. Figure 3-26 also depicts the numerous en route data entries used on a flight progress strip, generated by the ARTCC computer. Climbing, level flight, and descending during the en route phase of IFR flight involves staying in communication with ATC, making necessary reports, responding to clearances, monitoring position, and staying abreast of any changes to the airplane’s equipment status or weather.

Pilot/Controller Expectations

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 you receive a term “climb at pilot’s discretion” in the altitude information of an ATC clearance, it means that you have the option to start a climb when you wish, that you 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 “at pilot’s discretion” nor imposed any climb restrictions, you should climb promptly on acknowledgment of the clearance. Climb at an optimum rate consistent with the operating characteristics of your aircraft to 1,000 feet below the assigned altitude, and then attempt to climb at a rate of between 500 and 1,500 feet per minute until you reach your assigned altitude. If at anytime you are unable to climb at a rate of at least 500 feet a minute, advise ATC. If it is necessary to level off at an intermediate altitude during climb, advise ATC.

“Expedite climb” normally indicates you should use the approximate best rate of climb without an exceptional change in aircraft handling characteristics. Normally controllers will inform you of the reason for an instruction to expedite. If you fly a turbojet airplane equipped with afterburner engines, such as a military aircraft, you should advise ATC prior to takeoff if you intend to use afterburning during your climb to the en route altitude. Often, the controller may be able to plan traffic to accommodate a high performance climb and allow you to climb to the planned altitude without restriction. If you receive an “expedite” clearance from ATC, 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 ten minutes. After handoff to Center, we understood and read back, ‘Leaving 5,000 turn left heading 240° for vector on course.’ The First Officer turned to the assigned heading climbing through 5,000 feet. At 5,300 feet Center advised assigned altitude was 5,000 feet. We immediately descended to 5,000. Center then informed us we had traffic at 12 o’clock and a mile at 6,000. After passing traffic, a higher altitude was assigned and climb resumed. We now believe the clearance was probably ‘reaching’ 5,000, etc. Even our readback to the controller with ‘leaving’ didn’t 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 cockpit, the words “reaching” and “leaving” sound much alike.

For altitude awareness during climb, professional 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 to go” and “one to go.” Climbing through the transition altitude (QNH), both pilots set their altimeters to 29.92 inches of mercury and announce “2992 inches” (or ‘standard,’ on some airplanes) 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 inflight time often is flown in level flight at cruising altitude, from top of climb to top of descent (TOD). Generally, TOD is used in airplanes with a flight management system (FMS), and represents the point at which descent is first initiated from cruise altitude. FMSs 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. Descent planning is discussed in more detail in the next chapter, “Arrivals.”