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When Do Airplanes Begin Approach?  
User currently offlineVictorTango From India, joined Jan 2005, 500 posts, RR: 0
Posted (8 years 1 month 3 weeks 2 days 11 hours ago) and read 5842 times:

Hi Everybody?

This might be a vague question....

When do pilots begin their approach into the arrival airport? I mean at what distance from the airport and perhaps even altitude.

Infact does an approach mean that the aircraft has recieved it's final landing clearance from the tower?

Or does it mean that it just begins to line itself to the arrival runway?

Feel free to correct me if I'm unclear.

Many Thanks,

Olly

26 replies: All unread, showing first 25:
 
User currently offlineRedcordes From United States of America, joined Jan 2006, 245 posts, RR: 0
Reply 1, posted (8 years 1 month 3 weeks 2 days 9 hours ago) and read 5820 times:

The ILS is usually intercepted and a turn to "final" on the runway heading is made outside the "outer marker" radio beacon which is located 5 or 6 miles out. However at busier airports there are STAR's which are arrival routes that can begin a couple hundred miles from the airport to control the inbound traffic. I believe there are also transition routes that lead from the airways to the beginning of the STAR's. Final clearance to land might not be received until late in the final approach due to other traffic in the air or on the ground. Maybe one of the pros could elaborate/correct this.


"The only source of knowledge is experience." A. Einstein "Science w/o religion is lame. Religion w/o science is blind."
User currently offlineTimz From United States of America, joined Sep 1999, 6800 posts, RR: 7
Reply 2, posted (8 years 1 month 3 weeks 1 day 22 hours ago) and read 5728 times:

Yeah, it depends on what you mean by "approach". For most of the flight, airliners are cleared to ascend to or descend to various altitudes; you could say the "approach" begins when they're cleared for the approach-- that is, when they're cleared to descend all the way to airport level.

But they get that clearance from Approach Control-- in the US anyway. They still have to get landing clearance from the tower.


User currently offline777WT From United States of America, joined Jan 2005, 875 posts, RR: 1
Reply 3, posted (8 years 1 month 3 weeks 1 day 21 hours ago) and read 5713 times:

Modern jets have FMC which calculates TOC (Top of Climb) and TOD (Top Of Descent) which are based on alot of things such as Cost Index, cruising alt. and speed.

User currently offlineGoldenshield From United States of America, joined Jan 2001, 5970 posts, RR: 14
Reply 4, posted (8 years 1 month 3 weeks 1 day 21 hours ago) and read 5706 times:

Quoting 777WT (Reply 3):
Modern jets have FMC which calculates TOC (Top of Climb) and TOD (Top Of Descent) which are based on alot of things such as Cost Index, cruising alt. and speed.

Those are only used as references, and the the actual TOD will depend on airspace allowances. Also, in older, or less sophisticated, aircraft, the TOD is calculated by hand, but even then, it's still only a paper figure.



Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 5, posted (8 years 1 month 3 weeks 1 day 19 hours ago) and read 5670 times:

Quoting Timz (Reply 2):
But they get that clearance from Approach Control-- in the US anyway. They still have to get landing clearance from the tower.

Only where applicable. Granted, most airliners fly into busy airports with full-time control towers and approach radar facilities. Not all of them though. EGE is a good example; from midnight to 6AM, it's Class E airspace, and there's regular 737 service, along with turboprop and RJ service and a whole mess of private jets. In good visibility, Denver Approach will clear inbound a/c for a visual approach outside of the Class D boundaries (during tower operating hours). Late at night, airliners use the CTAF just like at small airports. It's not common, but it does happen.



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User currently offlineMir From United States of America, joined Jan 2004, 21530 posts, RR: 55
Reply 6, posted (8 years 1 month 3 weeks 1 day 17 hours ago) and read 5640 times:

Technically, the approach starts at the Initial Approach Fix (IAF), which is published on the charts. Not all approaches have an IAF though - some require that the airplane be vectored to the final approach course. Even if there is an IAF, ATC may decide to vector a plane to the final approach course anyway (this is how it's done 95% of the time - it makes for more efficient use of airspace).

However, by the time you get to the IAF, or start recieving your vectors to final, you should have everything set up for the approach and the appropriate briefings completed.

Quoting Redcordes (Reply 1):
I believe there are also transition routes that lead from the airways to the beginning of the STAR's.

There are, but they are referred to as part of the STAR, and will be found on the chart for the STAR.

For example, on the GOPHER 5 STAR into MSP (chart at http://map.aeroplanner.com/plates/FaaPlates_pdfs/00264GOPHER.PDF ), while the main part of the STAR starts at GOLFF (indicated by the bolder arrows), the FAR, AXN and BRD transitions are considered part of it as well.

Quoting Redcordes (Reply 1):
Final clearance to land might not be received until late in the final approach due to other traffic in the air or on the ground.

Final clearance to land is given upon intial contact with the tower 99% of the time, even if there is other traffic that will land before you. The only reason they may withhold it temporarily is if there is a potential separation conflict that needs to be worked out. At least that's how it is in the US, in Europe I believe that the preceeding aircraft has to be clear of the runway before landing clearance can be given - landing clearances on short final are pretty regular over there.

And, as Bri2k1 said, if you're heading into an uncontrolled field, ATC will terminate your radar service shortly after you're established on the approach, and it's up to you to self-announce on the CTAF.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlineFlyMatt2Bermud From United States of America, joined Jan 2006, 563 posts, RR: 7
Reply 7, posted (8 years 1 month 3 weeks 16 hours ago) and read 5457 times:

Quoting Bri2k1 (Reply 5):
Late at night, airliners use the CTAF just like at small airports. It's not common, but it does happen.

I recall an experience in the early 80's where an air carrier B-737 was late arriving into (KLEX) Lexington, Kentucky. The tower closed at midnight and airport had just installed pilot controlled lighting and it wasn't until the second ILS approach that the lights were activated by the crew for landing. The weather was reported 800 Overcast with 2 miles in Fog. With the local tower and local ATC closed, flights below 8,000 were non radar and our flight was holding at 5,000. The air carrier flight after the first approach flew the missed approach up to 4,000 and was then cleared back to the L.O.M. for their second approach. We were talking to the other crew on CTAF and assisted them with the lighting procedure. By clicking '7' times on the CTAF the runway lights are set to the highest intensity and the approach Runway End Indentifier Lights (REIL Strobes) are activated along with the High Intensity Approach Lighting System (ALSF-1) lead in lights. To put the lights to Medium intensity takes '5' clicks and Low intensity '3' clicks. Lighting system operations vary from airport to airport.

HM



"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci
User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
Reply 8, posted (8 years 1 month 3 weeks 15 hours ago) and read 5451 times:

Subtract the altitude of the airport or traffic pattern from your cruising altitude and divide by 3. That's roughly how far you should start your approach. Or multiply your altitude while omitting the last 3 zeros by 3, that tells your distance to reach the airport with a normal approach from your altitude. You get to know the number 3 real well when flying. I have no experience in IFR but I think in IFR flights, ATC tells you when to descend. Also most commercial planes have a flight management computer that basically gives the pilots all the navigation and calculations for that flight. All they do is type on the computer and the plane flys itself.

User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 9, posted (8 years 1 month 3 weeks 13 hours ago) and read 5432 times:

Quoting AirWillie6475 (Reply 8):
All they do is type on the computer and the plane flys itself.

This is not accurate. It's "flies," and the plane does not do this itself.

Quoting AirWillie6475 (Reply 8):
Subtract the altitude of the airport or traffic pattern from your cruising altitude and divide by 3. That's roughly how far you should start your approach.

Hm, OK. So, I cruise at 9,500 feet to BCT, elevation 13 MSL. I go for a straight-in approach, so I will round the target elevation to 0 and assume I need to descend the entire 9,500 feet. 9,500/3 = 3,167. So, do I start my descent just over half a mile from the airport? If I'm cruising at 120KT, or 2NM/min, that gives me about 15 seconds to descend almost 10,000 feet, or about 40,000FPM vertical speed. Straight down. And 40,000FPM works out to around 400KT, which would be frowned upon below 10,000MSL, especially straight down.

Quoting AirWillie6475 (Reply 8):
Or multiply your altitude while omitting the last 3 zeros by 3, that tells your distance to reach the airport with a normal approach from your altitude.

OK, I'll try it this way. I'm going to assume by "omit the last 3 zeros" you mean use the altitudes in thousands of feet. VFR flights, as you know, cruise at altitudes ending in 500, and few altimeters measure significant digits on the right side of the decimal, so I think it's a safe assumption. So again, from 9,500 feet about the airport, or 9.5 thousand feet, 9.5 x 3 = 28.5, a pretty rediculous distance (in feet) to try to descend those 9.5 thousand feet.

Moral: Post the entire formula, including unit conversions, or use an example.

Quoting AirWillie6475 (Reply 8):
in IFR flights, ATC tells you when to descend.

You could get a copy of the AIM and read it. You can be cleared to descend "at pilot's discretion:" ATC has offered the pilot the option of starting the climb or descent whenever he/she wishes and conducting it at any rate he/she wishes. He/she may temporarily level off at any intermediate altitude. However, once he/she has vacated an altitude, he/she may not return to that altitude. This is beneficial because it allows the pilots to stay at an economical and fast cruise altitude for a longer period of time, then descend faster, which keeps noise closer to the airport environment, without ATC needing to know the specific optimum altitudes and descent paths for every different type and loading situation.

The magical computer does not just pop out the number. After all, like any computer (including FBW control systems), garbage in = garbage out.

I'm willing to bet the OP has been on a commerical flight where a FA has said "Folks, we've been cleared to land at XXY and will be starting our final descent." This is pure hogwash. In Europe, controllers may not clear a plane to land on a runway until it is immediately available for their use, so late landing clearances are common. In the US, a landing clearance may be issued in conjunction with a spacing restriction, such as "Cleared to land #2" meaning once the aircraft in front of you has landed and exited the runway, you may then land. The point is the actual descent from cruise may take a long time, depending on traffic and airplane limits, 30-45 minutes is not uncommon. Most approaches are not conducted at maximum rate of descent. The "final approach" begins a few miles from the airport, when the plane descends from the traffic pattern altitude (at which it may or may not have been level for a while) to complete the landing.



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User currently offlineHighflyer9790 From United States of America, joined Jul 2005, 1241 posts, RR: 0
Reply 10, posted (8 years 1 month 2 weeks 6 days 21 hours ago) and read 5347 times:

Quoting Bri2k1 (Reply 9):
Most approaches are not conducted at maximum rate of descent.

so then what would be a typical descent rate from say, FL380 140nm out? or just a general descent rate from cruising altitude?



121
User currently offlineFlyMatt2Bermud From United States of America, joined Jan 2006, 563 posts, RR: 7
Reply 11, posted (8 years 1 month 2 weeks 6 days 21 hours ago) and read 5345 times:

Quoting VictorTango (Thread starter):
what would be a typical descent rate from say, FL380 140nm out?

We set 3% and Mach .85 in the FMS. ATC does not always give us what we want but today flying into Bermuda they did and we started our uninterrupted descent out of FL510 at 170 miles out.



"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci
User currently offlineSaturn5 From United States of America, joined Apr 2006, 313 posts, RR: 0
Reply 12, posted (8 years 1 month 2 weeks 6 days 20 hours ago) and read 5336 times:

Quoting FlyMatt2Bermud (Reply 11):
our uninterrupted descent out of FL510

FL510  confused 
what kind of aircraft do you fly ?????


User currently offlineHighFlyer9790 From United States of America, joined Jul 2005, 1241 posts, RR: 0
Reply 13, posted (8 years 1 month 2 weeks 6 days 20 hours ago) and read 5336 times:

Quoting FlyMatt2Bermud (Reply 11):
We set 3% and Mach .85 in the FMS. ATC does not always give us what we want but today flying into Bermuda they did and we started our uninterrupted descent out of FL510 at 170 miles out.

so how many feet per minute? my guess is that you were flying a gulfstream or citiation??  biggrin  lucky you! how bout an aircraft like a 757?



121
User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 14, posted (8 years 1 month 2 weeks 6 days 20 hours ago) and read 5332 times:

You can do rough calculations. If the airport is, say, 1000 feet, and pressure levels are close to standard, that means a 37,000 foot descent. Assume (unrealistically) you're 140NM out and the whole descent is at 250KT. A knot is a NM per hour, so you've got roughly 140/250*60=34 minutes for the descent. 37000 feet divided by 34 minutes gives you about 1100 FPM, if it was constant from the top of the descent to touchdown. Most descents are probably not going to start that far out, usually closer to 100NM for Class B airports in the US. Descent rates could be 1000FPM or less, or 2500FPM or more, depending on the situation. At the point of touchdown, it goes from whatever it is to 0, so you want it pretty low (like around 300). FlyMatt2Bermud's descent from an assumed 51,000 feet to sea level from 170NM is not typical, but sounds like it would have taken about 40 minutes at around 18000FPM using some of the same assumptions from above. If he'd tell us how long it actually took, we could get an idea of how close those assumptions get us to real-world figures...


Position and hold
User currently offlineHighFlyer9790 From United States of America, joined Jul 2005, 1241 posts, RR: 0
Reply 15, posted (8 years 1 month 2 weeks 6 days 20 hours ago) and read 5321 times:

Quoting Bri2k1 (Reply 14):
Most descents are probably not going to start that far out, usually closer to 100NM for Class B airports in the US.

actually, most of the time (thanks to the onboard map to view in the PTV) we start our descent about 140nm out in the 747...both while descending into BOS and EGLL...not to mention once in an AF A340 we started down from FL380 160nm out...



121
User currently offlineSaturn5 From United States of America, joined Apr 2006, 313 posts, RR: 0
Reply 16, posted (8 years 1 month 2 weeks 6 days 19 hours ago) and read 5315 times:

Quoting Bri2k1 (Reply 14):
Most descents are probably not going to start that far out, usually closer to 100NM for Class B airports in the US.

That is not true because in todays ATC environment rarely aircraft can descend continuously from its TOD to the airport, and definitely not towards larger class B airports. Sector controllers will usually have to obey so called LOAs for arrival traffic at larger airports - bringing you down in successive steps (so they hand you to the next controller at prescribed altitude) and this will significantly extend your total descent.


User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
Reply 17, posted (8 years 1 month 2 weeks 6 days 19 hours ago) and read 5315 times:

Quoting Bri2k1 (Reply 9):
Hm, OK. So, I cruise at 9,500 feet to BCT, elevation 13 MSL. I go for a straight-in approach, so I will round the target elevation to 0 and assume I need to descend the entire 9,500 feet. 9,500/3 = 3,167. So, do I start my descent just over half a mile from the airport? If I'm cruising at 120KT, or 2NM/min, that gives me about 15 seconds to descend almost 10,000 feet, or about 40,000FPM vertical speed. Straight down. And 40,000FPM works out to around 400KT, which would be frowned upon below 10,000MSL, especially straight down.

Quoting AirWillie6475 (Reply 8):
Or multiply your altitude while omitting the last 3 zeros by 3, that tells your distance to reach the airport with a normal approach from your altitude.

OK, I'll try it this way. I'm going to assume by "omit the last 3 zeros" you mean use the altitudes in thousands of feet. VFR flights, as you know, cruise at altitudes ending in 500, and few altimeters measure significant digits on the right side of the decimal, so I think it's a safe assumption. So again, from 9,500 feet about the airport, or 9.5 thousand feet, 9.5 x 3 = 28.5, a pretty rediculous distance (in feet) to try to descend those 9.5 thousand feet.

Well you have to have some common sense when you fly! On your fist example it's not 3167feet, it's about 31 MILES. On your second example it's 28.5 miles. We're talking about distance here.

[Edited 2006-06-14 02:46:47]

User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
Reply 18, posted (8 years 1 month 2 weeks 6 days 19 hours ago) and read 5304 times:

Quoting Bri2k1 (Reply 9):
You could get a copy of the AIM and read it. You can be cleared to descend "at pilot's discretion:" ATC has offered the pilot the option of starting the climb or descent whenever he/she wishes and conducting it at any rate he/she wishes.

Well yea they still tell you to descent. I know that ATC is at the pilot's service not the other way around so the pilot can do whatever he wants. And I don't know about all the FMCs but some do tell you everything about descent rates and approach distances.


User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 19, posted (8 years 1 month 2 weeks 6 days 18 hours ago) and read 5288 times:

Quoting AirWillie6475 (Reply 18):
I know that ATC is at the pilot's service not the other way around so the pilot can do whatever he wants.

It's this kind of statement that demonstrates what you really do and don't know.



Position and hold
User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
Reply 20, posted (8 years 1 month 2 weeks 6 days 12 hours ago) and read 5248 times:

Bri2k1, do you fly? It doesn't sound like it. Quit reading from the AIM/FAR manual.

User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 21, posted (8 years 1 month 2 weeks 6 days 6 hours ago) and read 5202 times:

Quoting AirWillie6475 (Reply 20):
Bri2k1, do you fly? It doesn't sound like it.

Hmm...Your profile lists your age as 16-20, and your occupation as Pilot. Let's think about this for a minute. The minimum qualifications for a Commerical Pilot Certificate include 250 hours of flight time, if training at a Part 61 school. Earlier, you said

Quoting AirWillie6475 (Reply 8):
I have no experience in IFR

which means your commercial certificate only allows you to fly during the day, in IMC, and within 50NM of the departure point, when flying for hire. If you were somehow able to accumulate the necessary 250 hours between your 17th birthday (when you were elegible for a Private Pilot Certificate) and today, while also being a student (but clearly not of an aviation school, when you make comments like the one in Reply #18), it would be remarkable. In fact, nearly impossible. So, let me ask you:

Quoting AirWillie6475 (Reply 20):
do you fly? It doesn't sound like it.

Now, let me refresh in your mind some of the rules of this forum.

Quote:
  • Rule #7: You agree that you will not use this discussion forum to post any statement which is knowingly false, inaccurate, abusive, vulgar, hateful, harassing, obscene, profane, graphic, threatening, invasive of a person's privacy or otherwise in violation of any law.
  • Rule #8:If you have something to say, please do so; however be sure to mention your sources, perhaps with an HTML link or reference to a publication. If you are merely providing an opinion, please MENTION THIS in your post. We would like to avoid arguments based on rumors or misinformation.

So, let me ask you -- in reply #8, when you posted two different formulae, without correct unit conversions or examples, and then in reply #17 told me

Quoting AirWillie6475 (Reply 17):
Well you have to have some common sense when you fly!

could that be construed as in violation of rules #7 and #8? I think so, which is why (in reply #9) I invited you to clarify your post, but you declined. Instead, you challenged my assertion that I am a pilot. I have clarified beyond a doubt in the past that I am indeed a current FAA-certificated pilot. I follow the FARs, including the requirement for the PIC to "become familiar with all available information concerning the flight," which includes a thorough knowledge of the AIM. For you to even insinuate that any pilot should do otherwise confirms in my mind that you are

  • NOT a pilot
  • NOT respectful of others, including pilots and air traffic controllers
  • NOT contributing anything to this thread

Your statements are so wildly inaccurate and without sources, so I do not consider you to be a threat to those who really want to learn. I offer you the advice of turning around your command in reply #20 and read a current FAR/AIM yourself; you would then be able to at least cite sources for your posts. I cite this again, as it's the single most factually inaccurate statement I've seen posted in a very long time:

Quoting AirWillie6475 (Reply 18):
I know that ATC is at the pilot's service not the other way around so the pilot can do whatever he wants.

For these reasons I've cited, my interaction with you stops here.



Position and hold
User currently offlineFlyMatt2Bermud From United States of America, joined Jan 2006, 563 posts, RR: 7
Reply 22, posted (8 years 1 month 2 weeks 6 days 4 hours ago) and read 5177 times:

Quoting Saturn5 (Reply 12):
what kind of aircraft do you fly ?

A Bombardier BD-700 1A11, Global Express (ours is the Global 5000 derivative)

Quoting HighFlyer9790 (Reply 13):

so how many feet per minute? my guess is that you were flying a Gulfstream or Citation?

The descent on the Global 5000 is typically set initially at 1200 fpm through FL440 then we can join the 3% slope which brings us down at 3,000 fpm without having to deploy flight spoilers and still not descending to steep for passenger comfort.

We used to operate a Citation 650 up to FL510, but it seldom ever climbed above 49,700 unless it was ISA -15 or so and the aircraft had crew only no payload.



"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci
User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
Reply 23, posted (8 years 1 month 2 weeks 5 days 20 hours ago) and read 5114 times:

Bri2k1

Quoting Bri2k1 (Reply 21):
Your statements are so wildly inaccurate and without sources, so I do not consider you to be a threat to those who really want to learn. :

Bri2k1, you need to get laid, I don't think any pilot around the local airport would know all of what you have just stated from AIM. Not everybody cares about all the little details of AIM. But if you think that my statements about how to calculate approaches are WILDLY inaccurate I suggest you talk to your local CFI and ask for clarification, see if I'm wrong. Also don't take things so literally. Like when it says occupation, I'm not a COMMERCIAL PILOT dude. I'm a college student. That's why I hate these scales they have here at A.Net. We shoud be able to put our actual age so detail oriented people like you don't take things the wrong way.


User currently offlineMir From United States of America, joined Jan 2004, 21530 posts, RR: 55
Reply 24, posted (8 years 1 month 2 weeks 5 days 14 hours ago) and read 5072 times:

Quoting AirWillie6475 (Reply 18):
I know that ATC is at the pilot's service not the other way around so the pilot can do whatever he wants.

Hardly. On an IFR flight, the pilot is at the beck and call of ATC, with the caveat that ATC cannot force a pilot to do something that is unsafe, illegal, or not physically possible. You don't have to accept an ATC clearance, but you'd damn well better have a good reason for doing so, otherwise the consequences will be severe.

Case in point: I was flying on Monday in the NYC area, which obviously is pretty busy. I had filed to cruise at 5,000, but ATC assigned me 7,000 in my pre-departure clearance. After takeoff, I asked departure if I could stay at 5,000 instead of climbing all the way to 7,000. He said (and I quote) "NO, climb and maintain 7,000". That left zero room for argument on the subject - he didn't care how much I wanted to be at 5,000, he wanted me at 7,000. Could I have said "unable" and forced him to give me 5,000? Yes. Would I have the FAA showing up at my door asking me to hand over my license for a month or two? Most likely.

Besides, it's not wise to give New York controllers a hard time. They have no qualms about reciprocating.  Smile

Quoting AirWillie6475 (Reply 17):
Well you have to have some common sense when you fly! On your fist example it's not 3167feet, it's about 31 MILES. On your second example it's 28.5 miles. We're talking about distance here.

Fine. Let's talk about distance. You said to divide the amount of feet you need to lose by 3. Let's say you need to lose 7,000 feet. 7,000/3=2,333. 2,333 miles? Sounds a bit excessive. 2,333 feet? Yeah, I could descend 7,000 feet in that short amount of space, but I wouldn't be able to bring the plane with me. So what units are we talking about? How do you get 31 miles from 3,167 feet? And once you've established which distance you're using, how do you know how fast to descend (this is pretty important, since you're going to have to start down a lot sooner if you plan on going down at 500fpm rather than 1500fpm)?

Here's the formula I use: I start with a 1000fpm descent rate, which is nice both from a comfort standpoint and ease-of-mental-math standpoint. I then take my groundspeed rounded to the nearest ten, chop off the last zero and divide by six. The result is about how many miles I need for every thousand feet I want to descend.

For example, if I need to cross a certain point at 3,000, and I am now at 10,000 with a groundspeed of 110kts, all I need to do is divide 11 by 6 (about 1.8), and then multiply 1.8 by 7 (about 12.5). So, start descent 12.5 miles out. I generally add a few miles to that number so I can be sure of not arriving too high.

The other way is to multiply the number of thousands of feet you need to lose by 3, and then multiply your groundspeed by six to come up with your required descent rate. Using the same situation as above, I'd do 7*3=21 miles, and 110*6=660fpm as a descent rate. Again, add a few miles to make sure that you get down in time.

The second formula is easier, but I use the first since I'd rather stay at altitude longer, and the planes I fly now are slow enough that maintaining 1000fpm on the way down won't require me to start down a long ways out. Were I flying a jet at 450kts groundspeed and needed to descend 35,000 feet, I'd need to start my descent 262 miles out with the first formula - way too much. Using the second one, I'd start 105 miles out and descend at 2700fpm.

Besides which, I fly most of my descents at 120kias, which is generally close enough to my groundspeed that I can just use two miles for every thousand feet and come up with a good estimate.

One can also tailor the first formula to suit your desired descent profile (2,000fpm instead of 1,000, for instance).

-Mir



7 billion, one nation, imagination...it's a beautiful day
25 YYZSaabGuy : People..........relax. One of the joys of the Tech Ops forum (as opposed to, y'know, that OTHER one) is that most members posting here are able to exc
26 AirWillie6475 : I agree, I want to use that Special Olympic joke about arguing on A.Net but I'd rather not.
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