Control of Aircraft — Page 343, Lesson 455

I want to walk you through the control of aircraft, specifically the holding and approach procedures that ATC uses to manage arriving traffic. Let's start with holding. Holding is what happens when ATC approves a request to delay an aircraft, usually for weather improvement, though it can be for other reasons too. So an aircraft circles at a designated fix, burning time until conditions improve. Now here's a key operational detail: if other aircraft in the holding pattern decide to make an approach and radar is available, a pilot who chooses to remain holding will be vectored to an adjacent fix to continue holding there. Alternatively, that pilot may be vectored, or given a procedural clearance, to place the aircraft at the top of the stack. The stack is the vertical column of aircraft holding at the same fix at different altitudes. Putting one aircraft at the top of the stack frees up the lower levels so other aircraft can carry out the approach procedure and land. Next is credit time. When an aircraft has been authorized to absorb delay time while en route — either by reducing cruising speed or by holding en route — that delayed time should be credited in any stacking. In other words, if you've already absorbed some delay earlier in the flight, that time counts toward your holding time, so you shouldn't be made to wait the full amount again in the stack. Now let's move to timed approaches, paragraph 17.39. Timed approaches allow subsequent aircraft to commence approaches more frequently than the normal spacing specified in paragraph 17.38. Here's how it works: an aircraft is cleared to depart the fix of the stack a set period of time after the preceding aircraft. So instead of waiting for the aircraft ahead to land and clear the runway, you're released from the fix on a time interval. This procedure must be authorized by the authority, and there are two requirements you must comply with. First, a suitable point on the approach path must be specified as a check point for timing the successive approaches. That point has to be capable of being determined by the pilot — for example, a VOR radial or a DME range. VOR is the VHF omnidirectional radio range, and DME is distance measuring equipment, so the pilot can positively identify that exact position. Second, each aircraft is given a time at which to pass that specified point inbound. The purpose of that time is to achieve the desired interval between successive landings on the runway, while respecting the applicable separation minima at all times, including the runway occupancy period. Runway occupancy period is the time an aircraft spends on the runway from touchdown until it clears. The time determined must be passed to the pilot early enough to allow sufficient time for the pilot to arrange the flight to comply with it. Finally, paragraph 17.40 introduces the Expected Approach Time, abbreviated EAT. An expected approach time shall be determined for an arriving aircraft that will be subjected to stacking. So if you're going to be held in the stack, ATC must give you an EAT — the time you can expect to be cleared to commence your approach. That's the core of it: holding, credit time, timed approaches, and the expected approach time.