POINTS TO PONDER:
The guiding principle of a crossing restriction is that the last ATC clearance has precedence over the previous clearance.
When the route or altitude is amended, the controller will restate altitude restriction.
“If altitude to maintain is changed or restated, whether prior to departure or while airborne, and previously issued altitude restrictions are omitted, those altitude restrictions are canceled.”
HOW TO MEET A CROSSING RESTRICTION:
Miles to descend to meet crossing restriction (3-degree descent) — altitude to lose (in thousands) times 3 plus 10 percent.
Three-degree rate of descent — halve the ground speed and add a zero.
Rate of descent — altitude to lose divided by time to fix.
To simplify for most jets — determine minutes from the fix (groundspeed divided by 60, or just glance over at the GPS or FMS); plan on descending at 2,000 fpm.
Your groundspeed at FL 310 is 420 knots, and you are cleared to cross 30 miles from the VOR at 10,000 feet and 250 kias. How far out would you need to start the descent?
a. It’s common for most jets to descend at idle power using the 3-to-1 rule (i.e., 3 miles for every 1,000 feet of altitude to lose).
b. The altitude to lose is 21,000 feet. Descending from FL 310 to 10,000 feet would probably require a 3,000 fpm descent in order to maintain the advantage of the higher altitude for a longer period of time. 420 knots is 7 miles per minute (420 ÷ 60).
c. 21,000 feet ÷ 3,000 fpm = 7 minutes.
d. 7 minutes x 7 miles per minute = 49 miles are required to descend the 21,000 feet.
e. Add about 10 miles to comfortably slow to 250 kias.
f. 49 + 10 + 30 (from fix) = start down 89 miles from the VOR.
Of course, by far the most popular method is to simply enter the VNAV information into the FMS and let it and the autopilot take care of all that annoying thinking.
Speed Adjustment Terminology
Resume normal speed — Used to terminate ATC-assigned speed adjustments on segments where no published speed restrictions apply. It does not cancel published restrictions on upcoming procedures. Comply with speed restrictions — Used when the aircraft is joining or resuming a charted procedure or route with published speed restrictions.
Comply with restrictions — Requires compliance with all altitude and/or speed restrictions depicted on the procedure.
Delete speed restrictions — Used when ATC-assigned or published speed restrictions on a charted procedure are no longer required. This does not relieve the pilot of speed restrictions applicable to 91.117.
Resume published speed — Issued to terminate a speed adjustment where speed restrictions are published on a charted procedure. When instructed to “comply with speed restrictions” or to “resume published speed,” ATC anticipates pilots will begin adjusting speed the minimum distance necessary prior to a published speed restriction so as to cross the waypoint/fix at the published speed.
If you’re flying a jet — You can often expect to be instructed to cross 40 nm from the destination airport at either 11,000 or 12,000 feet with a possible speed restriction of 250 knots.
If you’re flying a prop airplane — In this case, you can often expect to cross 40 nm from the destination airport at 7,000 feet for turboprops and 4,000 feet for piston airplanes. Turboprop pilots, don’t forget to slow to 250 knots before descending below 10,000 feet.
If you’re headed for an airport not served by Approach Control, you can expect simple step descents with no crossing restrictions. You can request a descent from Center 60 miles out or wait a bit longer.