The Crossing Restriction You Cannot Make
You are N12345, level at FL240, 42 NM from the HARRS intersection at 300 kt groundspeed. ATC: 'N12345, cross HARRS at and maintain eight thousand, expect six thousand one zero miles after HARRS.' Your descent data: 1,300 fpm at 300 kt; 1,800 fpm at 250 KIAS (260 kt groundspeed); no speedbrakes. The MEA after HARRS is 6,400 ft and the minimum vectoring altitude is 6,000. There is a broken layer at 12,000 ft. The controller is working three other aircraft and the frequency is congested.
1. What must you read back, and what must you work out before you key the microphone?
Read the worked answer
Read back the restriction and the altitude; do the arithmetic first.
Mandatory readback (§91.123 and AIM 4-4-7) covers altitude assignments, headings/vectors, and crossing restrictions: 'Cross HARRS at eight thousand, expect six thousand one zero miles after HARRS, N12345.' The 'expect' is information, not a clearance - do not read it back as though it were one.
The arithmetic, before transmitting:
- Altitude to lose: 24,000 - 8,000 = 16,000 ft.
- Distance and time: 42 NM at 300 kt GS = 8.4 min.
- Required rate: 16,000 / 8.4 = ~1,900 fpm.
- Required gradient: 16,000 / 42 = 381 ft/NM; 381 x (300/60) = 1,900 fpm. Same answer, two ways.
You have 1,300 fpm at that speed. In 8.4 min you would lose only ~10,900 ft and cross HARRS at roughly 13,100 ft - 5,100 ft high. The clearance as issued is not flyable. That must be established now, not discovered at the fix.
2. You conclude you cannot make it. What do you say on this transmission, and what options can you offer the controller?
Read the worked answer
Say 'unable' now, with a request attached.
'Unable the crossing restriction at HARRS - request lower altitude now' (or 'request pilot's discretion descent,' 'request a vector for descent,' 'request a different crossing fix'). Controllers build the vertical profile assuming you will meet it. An inability discovered at HARRS is an altitude deviation and a loss of separation; the time to negotiate is eight minutes out, not at the fix.
Options to offer:
- Immediate descent from FL240. An extra 4,000-6,000 ft of head start usually solves the whole problem; a busy controller will nearly always find that easier than re-sequencing you.
- Slow to 250 KIAS. GS 260: 42/260 x 60 = 9.7 min; 1,800 x 9.7 = 17,460 ft, so it just works on paper. In practice deceleration from 300 to 250 kt costs 4-5 NM and 30-45 seconds of lost descent, so the margin is essentially zero. Do not accept that trade single-pilot on a congested frequency.
- If no relief is available, request a vector or a hold to lose altitude rather than diving at an unsafe rate.
Do not descend below 8,000 without a clearance, and note that after HARRS the MEA is 6,400 - the 'expect 6,000' is not usable until ATC clears it.
3. 'Descend at pilot's discretion, maintain 8,000' comes back. What does that authorize, and where is the trap?
Read the worked answer
It hands you the profile.
It authorizes descent to 8,000 beginning at a time you choose. You may level off temporarily, and ATC retains terrain and obstacle clearance along your route, so no crossing restriction is implied.
What it does not do:
- It does not authorize 6,000; 8,000 is a hard floor until amended.
- It does not relieve you of any later restriction. The discretion is about when you start, not about the end state.
The trap: if you dawdle because the air is smooth and then hear 'cross HARRS at 8,000' 15 NM later, you now own an impossible profile and have nobody to blame. Use the clearance deliberately - begin a descent immediately at a rate that keeps you slightly ahead of the ideal profile, so you retain the ability to steepen if a new restriction arrives. Keep the required rate in your head continuously: gradient (ft/NM) = altitude to lose / NM to go; rate = gradient x (GS/60). At 42 NM and 300 kt that is 1,900 fpm; at 20 NM it is 4,000 fpm - which is why the answer to 'can you make it?' changes by the minute.