Descents and Busy Frequencies 

Question: 
Does air traffic control (ATC) start an airliner's descent based on their flight plan, or is the pilot's planning for "back-up" purposes if ATC does not issue a clearance to start descending in a timely manner?  How often does your plan for descent coincide with actual ATC clearances?  Also, when arriving or departing out of heavily congested airspace and the frequencies are constantly swamped, how do you get a word in to let the controllers know what's going on beyond the scope of their radar screens?

Joe Carpenter, Ft. Meade, MD

Answer:
Air traffic control (ATC) doesn’t know what our planned point of descent (POD) is specifically per our flight plan, but they do start us down with regard to several factors having to do with how long it will take us to get down in terms of distance, traffic factors, etc.  Usually we don’t have to ask for a descent but once in awhile if we feel that they’ve forgotten or have been too busy to issue a descent clearance, we'll go ahead and ask for it, especially if there is a tailwind to consider, giving us less distance to get down due to our higher groundspeed.

Sometimes ATC will issue a descent before we're really ready for one.  If you listen to ATC long enough eventually you'll hear a pilot issued a descent, and the pilot respond with something like, "Can you give that to us with discretion?" or some other phrase including the word "discretion."  The pilot is asking the controller if it's okay to delay the descent just a bit for his/her own planning purposes.

Here's a realistic sample of what you might hear:  Delta 391 descend and maintain flight level (FL) 310.

Delta responds:  May we have that our discretion?

Controller:  Delta 391 descend now to flight level 350 then 310 your discretion.

Or the controller might say simply:  Affirmative, Delta 391 descend to and maintain flight level 310 at your discretion. 

Or, the controller might say:  Negative, Delta 391 descend now and maintain flight level 310.

At some destinations, like airports in the Washington DC area when we're inbound from Chicago or other points basically west, they bring us down early every time.  On one flight we were brought down early as usual, the frequency wasn't that busy and I finally asked the controller why they always brought us down so early in this area.  The controller responded that it was to avoid traffic conflicts with New York Center After I learned that I never again asked to delay my descent coming into the DC area as the request would always be denied for the same reason.

The point of descent (POD) on our flight plan is more than just a backup, as you've asked.  It is in fact our planned descent point.  Sometimes we descend exactly where the flight plan says we should, but all sorts of factors may dictate otherwise.

If we never reached our cruise altitude, for instance, due to traffic, wind, weather, a lousy ride reported at our planned altitude, etc., then our point of descent will obviously have to change.  Pilots often use a mental calculation known as a "3 to1 glide ratio."  Say we're at 40,000 feet.  Take the 40, multiply it by 3 and you have 120.  We know to plan our descent around 120 nautical miles from our destination and will add a little distance if we have a tailwind (due to our higher groundspeed) or subtract a few miles if we have a headwind.

Having said all this, a good deal of the time our descent is issued right where we expect it, or else we're given discretion to start down when we want and we can start down at the POD on our flight plan.

Regarding congestion on air traffic control frequencies, busy Chicago Approach is a wonderful example of where we might not be able to check in right away or otherwise get a word in when we need to.  However, if we're a little patient and just wait, from experience we know they're likely to call us even if we haven't yet had a chance to let them know we're there. If we have something extremely urgent, meaning a bona fide emergency, we can always broadcast on emergency frequency 121.5, which all air traffic control facilities monitor, but otherwise we just keep trying until we're heard.

Some readers may be wondering where they can listen to air traffic control.  Check out the two ATC links I have right on my home page in the center section at www.fromthecockpit.com.