8/15/19 AIM  is authorized to conduct the climb or descent at any rate, and to temporarily level off at any intermediate altitude as desired. However, once the aircraft has vacated an altitude, it may not return to that altitude. d. When ATC has not used the term “AT PILOT’S DISCRETION” nor imposed any climb or descent restrictions, pilots should initiate climb or descent promptly on acknowledgement of the clearance. Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate of between 500 and 1,500 fpm until the assigned altitude is reached. If at anytime the pilot is unable to climb or descend 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 or descent, advise ATC, except when leveling off at 10,000 feet MSL on descent, or 2,500 feet above airport elevation (prior to entering a Class C or Class D surface area), when required for speed reduction. 14 CFR Section 91.117. Leveling off at 10,000 feet MSL on descent or 2,500 feet above airport elevation (prior to entering a Class C or Class D surface area) to comply with 14 CFR Section 91.117 airspeed restrictions is commonplace. Controllers anticipate this action and plan accordingly. Leveling off at any other time on climb or descent may seriously affect air traffic handling by ATC. Consequently, it is imperative that pilots make every effort to fulfill the above expected actions to aid ATC in safely handling and expediting traffic. e. If the altitude information of an ATC DESCENT clearance includes a provision to “CROSS (fix) AT” or “AT OR ABOVE/BELOW (altitude),” the manner in which the descent is executed to comply with the crossing altitude is at the pilot’s discretion. This authorization to descend at pilot’s discretion is only applicable to that portion of the flight to which the crossing altitude restriction applies, and the pilot is expected to comply with the crossing altitude as a provision of the clearance. Any other clearance in which pilot execution is optional will so state “AT PILOT’S DISCRETION.” 1. “United Four Seventeen, descend and maintain six thousand.” 1. The pilot is expected to commence descent upon receipt of the clearance and to descend at the suggested rates until reaching the assigned altitude of 6,000 feet. 2. “United Four Seventeen, descend at pilot’s discretion, maintain six thousand.” 2. The pilot is authorized to conduct descent within the context of the term at pilot’s  discretion as described above. 3. “United Four Seventeen, cr oss Lakeview V − O − R at or above Flight Level two zero zero, descend and maintain six thousand.” 3. The pilot is authorized to conduct descent at pilot’s discretion until reaching Lakeview VOR and must comply with the clearance provision to cross the Lakeview VOR at or above FL 200. After passing Lakeview VOR, the pilot is expected to descend at the suggested rates until reaching the assigned altitude of 6,000 feet. 4. “United Four Seventeen, cross Lakeview V − O − R at six thousand, maintain six thousand.” 4. The pilot is authorized to conduct descent at pilot’s discretion, however, must comply with the clearance provision to cross the Lakeview VOR at 6,000 feet. 5. “United Four Seventeen, descend now to Flight Level two seven zero, cross Lakeview V − O − R at or below one zero thousand, descend and maintain six thousand.” 5. The pilot is expected to promptly execute and complete descent to FL 270 upon receipt of the clearance. After reaching FL 270 the pilot is authorized to descend “at pilot’s  discretion” until reaching Lakeview VOR. The pilot must comply with the clearance provision to cross Lakeview VOR at or below 10,000 feet. After Lakeview VOR the pilot is expected to descend at the suggested rates until reaching 6,000 feet. 6. “United Three Ten, descend now and maintain Flight Level two four zero, pilot’s discretion aft er reaching Flight Level two eight zero.” 6. The pilot is expected to commence descent upon receipt of the clearance and to descend at the suggested rates until reaching FL 280. At that point, the pilot is authorized to continue descent to FL 240 within the context of the term “at pilot’s discretion” as described above. f. In case emergency authority is used to deviate from provisions of an ATC clearance, the pilot − in − REFERENCE − NOTE − NOTE − EXAMPLE − NOTE − EXAMPLE − NOTE − EXAMPLE − NOTE − EXAMPLE − NOTE − EXAMPLE − NOTE − EXAMPLE − ATC Clearances and Aircraft Separation 4 − 4 − 6