ATC Climb or Descent Clearance – IFR and VFR
AIM 4-4-10 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.
a. The following reports should be made to ATC or FSS facilities without a specific ATC request:
1. At all times.
… (c) When unable to climb/descend at a rate of a least 500 feet per minute….
Instrument Departure Procedures
AIM 5-2-8 b.1 1. Unless specified otherwise, required obstacle clearance for all departures, including diverse, is based on the pilot crossing the departure end of the runway at least 35 feet above the departure end of runway elevation, climbing to 400 feet above the departure end of runway elevation before making the initial turn, and maintaining a minimum climb gradient of 200 feet per nautical mile (FPNM), unless required to level off by a crossing restriction, until the minimum IFR altitude. A greater climb gradient may be specified in the DP to clear obstacles or to achieve an ATC crossing restriction. If an initial turn higher than 400 feet above the departure end of runway elevation is specified in the DP, the turn should be commenced at the higher altitude. If a turn is specified at a fix, the turn must be made at that fix. Fixes may have minimum and/or maximum crossing altitudes that must be adhered to prior to passing the fix.
Implications for IFR flight
From the above information, it appears that enroute courses are designed with a minimum climb rate of 500 fpm. When approaching a fix with an altitude change, that does not have a Minimum Crossing Altitude, you may cross the fix at the current Minimum Enroute Altitude and then climb at least 500 fpm to the new MEA. Departure procedures assume that you will be climbing with at least 200 feet per nautical mile climb rate (unless a higher rate is specified). If you are complying with any minimum climb rates on a departure procedure, and you arrive at the fix that begins a portion of the DP with a higher MEA, then you may cross the fix at your current altitude and climb to the new MEA. You must climb at least 500 fpm or (if specified) at the rate in the DP. If you are unable to do so you must notify ATC.