top of descent

[van der vart]

hi guys

can someone pls answer the following questions

top of descent (NM) = 3 x 33 (if at FL 330). am i correct?

is it normal for controllers to ask pilots for their top of descent point? if so, what phraseology can i expect from them?

thanks

[Arkady]

UK Civil controllers will rarely ask for top of descent with Airways traffic as most destinations have level restrictions built into the inbound routing. The controller will usually give you a conditional clearance to be level at an intermediate level by a certain point, then transfer you to the next sector for further descent.

eg "BAW 123, when ready, descend FL xxx to be level by yyy"

[055166k]

Yes, I use 3 times FL as a rough guide...making a mental adjustment for strong wind. To assist the pilot I never omit giving the relevant STAR, where there is one published, because the STAR chart contains details of "level planning" to assist in preparation and cockpit briefing [ I hope ]. If the aircraft is descending at high IAS either at the request of ATC or its own preference I would add on 10 miles to slow down without having to use the "brakes".

[Lynx Boy]

van der vart,

I don't know whether you are a pilot or a controller, so I just wanted to put forward a few points in case you are a trainee or low-hours pilot or controller to avoid confusion.

There is no rule of thumb for descent point that covers all aircraft types because any calculation is dependent upon the aircraft's groundspeed and its rate of descent. I'm sure you can see that if two aircraft are at FL100 with one travelling at 400kt and one at 200kt, the aircraft at 400kt will have to begin its descent much further away from its destination to get down in time.

Rate of descent affects things in a similar way, but again different aircraft use different rates of descent. Most piston aircraft and smaller turboprops are unpressurised and use a maximum rate of descent of around 750 feet per minute. This is to avoid passenger discomfort (sore ears etc). Pressurised aircraft, however, such as jets and larger turboprops are not limited in this way and can have rates of descent much greater than this.

To (hopefully) answer your question, there are general rules of thumb in use (as confirmed by 055166k). Pilots will use the most appropriate for the aircraft type they are flying and I should imagine that controllers could get by using one for turboprops and one for big jets as they are, after all, providing only ballpark distances (can any controllers confirm or otherwise?).

I hope this is of use.

LB

PS I did note that your example used FL330 and this would only apply to jets but I wasn't sure if this was by chance or design.

[Timothy]

If you are calculating in your head there is only really one way to do it, which is to work out your time to descend (330 to SL at 2000 fpm = 17 min) then your miles per minute (so at 420kts you are doing 7 miles a minute), multiply the two together (7 x 17 = 120nm).

If you always descend at the same rate and the same speed, then you can combine the two into one fudge factor, thus if you descend at the above figure the fudge factor is to divide by 3 (=110 nm for F330) or multiply by 4 (=132 nm) [both good enough for government work] but you have to remember to adjust for ground speed (wind obviously makes a difference.)

Don't they do this stuff at pilots school any more? :{

W