Spoiler From Spain, joined Apr 2007, 0 posts, RR: 0
Reply 1, posted (14 years 13 hours ago) and read 2722 times:
Well, I've never flown in the flight levels (as a pilot anyway), But I'd imagine that PD might have something to do with severe weather avoidence. Some of those thunderstorm tops can go up to FL400. But like I said, I'm not really sure.
XFSUgimpLB41X From United States of America, joined Aug 2000, 4250 posts, RR: 37
Reply 2, posted (14 years 13 hours ago) and read 2715 times:
It is basically a term used when you have plenty of room to work with.... i get PD descents all the time down at the lower altitudes i use (12,000 and below usually)... I guess you could say it is the opposite of expediting. And yes, it is also used for weather/buildup avoidance...although in that case i am almost always told: "when able, climb/decend and maintain (insert altitude here)"
AAR90 From United States of America, joined Jan 2000, 3516 posts, RR: 45
Reply 3, posted (14 years 7 hours ago) and read 2682 times:
>I understand the definition of PD but WHY is it used?
As stated earlier, if plenty of airspace to work with the controller can issue a P.D. clearance when workload and radio traffic volumn permit.
>When is it used?
Most often sometime prior to normal top of descent point for a lower altitude crossing restriction.
>How often is it used?
IMHO, I'd guess as often as traffic and workload permit. Removes burden from ATC and places it upon pilot --who probably wants it so he can perform descent the way he wants rather than the way ATC wants.
Unless traffic volumn is real high, I see this just about every flight up/down west coast. Usually ATC issues PD clearance shortly after we "request lower."
*NO CARRIER* -- A Naval Aviator's worst nightmare!
N766AS From , joined Dec 1969, posts, RR:
Reply 4, posted (14 years 4 hours ago) and read 2653 times:
In my experience listening to Seattle Center, the controllers tell the pilots to 'descend pilots discretion and maintain FL240' so that the pilot can choose his descent rate and time to start down, keeping in mind the arrival restrictions. Often a pilot checks in on the next freq "Alaska XXX with you, pilots discretion FL240" and the controller will respond with "Alaska XXX, cross JAKSN at 16000, descend and maintain 12000, Seattle altimeter...blah blah blah".
So basically, from what I have learned, its just so that the pilot has a bit of freedom with his descent and the controller doesn't have to do the math in his/her head on when the start the aircraft down.
SkyWestPilot From , joined Dec 1969, posts, RR:
Reply 5, posted (13 years 12 months 3 days 16 hours ago) and read 2601 times:
N766AS, in your example above, it's the high altitude controller giving PD to FL240. He does this getting ready for the handoff to the next contrller working FL240 and below. I'm not sure if he flashes the aircraft to the next controller right away or when they start down. Like AAR90, I get PD clearances all the time flying in the Pac. NW. It's a workload issue on the controller's part. If there's room, it's easier to let the pilot plan his or her own descent.
CX flyboy From Hong Kong, joined Dec 1999, 6669 posts, RR: 55
Reply 6, posted (13 years 12 months 3 days 4 hours ago) and read 2594 times:
In modern airliners, the FMS calculates the top of descent point and marks it on the ND (Navigation Display). When this point is reached, as long as a lower altitude is dialied into the autopilot window, the aircraft will automatically retard the thrust levers and begin descent. If descent is initiated before this, then we will be below optimum descent profile.
A descent at pilots discretion before this point means that we can descend when we prefer to, as opposed to immediately.
N766AS From , joined Dec 1969, posts, RR:
Reply 7, posted (13 years 12 months 3 days 4 hours ago) and read 2584 times:
>It's a workload issue on the controller's part
Yeah, I think I was trying to get at that (but it was probably lost in all the other crap...hehe). I know that the controllers (at least the ones I listen to regularly) have a VERY high workload just about all of the time.
I have yet to drive down to Seattle Center (its only a few miles down the road) to watch them in action in person- I think that would be a great experience.