Take the 2-minute tour ×
Aviation Stack Exchange is a question and answer site for aircraft pilots, mechanics, and enthusiasts. It's 100% free, no registration required.

What rule of thumb would you use for your airplane insofar as how far from your destination you would begin your descent given an expected straight-in approach and landing in no-wind conditions and no ATC or traffic restrictions? How would your weight influence that rule of thumb, bearing in mind that the heavier you are, the more potential energy you have to dissipate?

With those same conditions, what rule of thumb would you use to determine how close to the airport you could get before descending and still come straight in if you had an empty airplane save for necessary fuel and if you were allowed to use any aircraft configuration you wanted, and what would that configuration be?

share|improve this question
    
If on an IFR approach, my understanding is the approach plates tell you these details (caveat: I'm not IFR rated). Flying VFR, my opinion is there's no reason that trumps safety to fly a straight-in approach and that an insistence on it just demonstrates a hazardous pilot attitude. It's much safer to slow things down in the pattern, giving you time to see the airport and other traffic (some of which might be NORDO) and time to properly plan your landing, even at airports you've never seen before. –  mah Feb 27 at 12:04
    
@mah It's fairly common when flying IFR, especially to smaller airports, for ATC to clear you direct to the airport and to give you a descent "at pilots discretion" . I'm this case, you will be landing straight in and planning your own descent. –  Lnafziger Feb 27 at 13:31
    
Terry, asking this "for your airplane" makes this question very broad, and there is no "one right answer". I would suggest changing it to "for a typical heavy jet" or whatever you would be most interested in. –  Lnafziger Feb 27 at 13:37
1  
@mah: The plates define the altitudes for the the approach (from IAF onward) and sometimes for final parts of the arrival (before IAF), but never from the en-route flight level. –  Jan Hudec Feb 27 at 14:16

4 Answers 4

For ATC planning of jet transport aircraft, 3NM per 1000 ft, plus 10 NM for deceleration is used as a rule of thumb.

$$FL300=3\times30+10=100NM$$

Most of the 10NM for deceleration will be consumed at around FL100.

Weight should not change the descent distance a lot, it mainly affects the speed. The lighter the slower.

share|improve this answer
1  
In addition to this rule of thumb, absent a crossing restriction into smaller fields, I would plan cross 10k feet approximately 30nm from the field if relatively straight in, closer if we were expecting a downwind leg. –  casey Feb 27 at 15:52

Well, you did say 'for your airplane'...

In a DA-42, with a cruise speed of 150kts, the game is to arrive at the FAF, at the intermediate approach altitude, at about 120kts. Or even better: be at that altitude and speed about 1NM before FAF so you still have 15 seconds to spare.

So:

  • 5NM for each FL/1000' to descend (500fpm at 2.5NM a minute)
  • +1 NM for each 10kts to slow down (usually it's 30kts so 3 NM). If I'm heavy(er) I'll use 1.5NM for each 10kts
  • +1 NM for each 10mb above 1013, or minus for lower pressure
  • +1 NM for spare

For example: FL90 to 2000' at 1023 hPa

That's 7 flight levels, so 35 NM, plus 3 miles to slow down, plus 1 mile for the pressure correction, plus 1 mile to spare, total is ~40 miles.

We're not pressurized so max descent is limited to 1000fpm: halving those 40NM to 20NM gives you the limit for a (semi-decent) slam dunk descent and landing, but it's not really a fair comparison in a light twin vs a big jet. With a gear down and full flaps this thing can descend over 4000fpm while still slowing down.

share|improve this answer
    
"With a gear down and full flaps" - well, the arrival is flown clean. And jets glide pretty well; typical L/D is around 18:1 which translates to 3NM per 1000ft. –  Jan Hudec Feb 27 at 14:25

Descent planning for jets can get a little tricky at times, and you have pointed out some of the things to look at when deciding when to start down (assuming that ATC gives you the option to choose your own top of descent).

The descent point is dependant on your aircraft and how much drag there is at your descent speed. Anything that adds more drag allows you to come down faster, but also makes the flight less efficient since you would have burned less fuel had you just started down earlier without the drag.

A typical jet will descend with the engines at flight idle and the speed at the recommended descent speed, which is usually close to the maximum speed for the airplane. A good rule of thumb for most jets would be:

3 miles for each 1,000 ft. of altitude to lose + 1 mile for every 10 KIAS of airspeed that you need to lose before landing.

Example:

We are cruising along at 38 thousand feet and landing at an airport with an elevation of 2 thousand feet which means that we need to lose 36,000 ft. Let's say that we will be descending at 350 KIAS and need to slow to 150 KIAS for landing:

Thousands of feet to lose: $38-2=36$ thousand feet
Tens of KIAS of airspeed to lose: $35-15=20$ tens of KIAS

Distance = $36\times3+20=128NM$ from your destination

So in this case, we will start down 128 miles out.

There are various factors that can cause this to be a little off, the biggest being the wind. As the descent progresses, you should mentally double check your progress. In this case, maybe check when we have 20,000 ft to go (should be 80 NM out now) and when we have 10,000 ft to go (should be 50 NM out now).

If one of your checks indicates that you are a little off, you can adjust your descent rate (adding a little power or using spoilers to descend more quickly) to get back on track.

A slight variation of this uses the same calculations, but instead of using the airport as the "target" altitude and speed, they use a point 30 miles from the airport at 10,000 ft above the airport at 250 KIAS.


The above method is enough to get someone started, and just "works" most of the time. As a pilot becomes more comfortable with the math, they can start to make adjustments for other factors like:

  • The effect of the winds at the various altitudes during the descent
    • Headwinds mean that you can wait a little longer to descend
    • Tailwinds mean that you should start down a little sooner
  • Weight of the aircraft
    • The lighter the aircraft, the later you can start your descent. (If this seems backwards, consider that when you are heavy it will push the airspeed higher faster as you lower the nose and you will have to raise it to keep from overspeeding. When light, you can keep the nose further down and will therefore have a higher descent rate.)
  • The direction that you are approaching the airport from
    • If you are landing straight in you should start sooner
    • If you are going to fly a downwind, base, and final, you can start a little later
  • ATC speed restrictions can prevent you from descending at your normal rate, so you may need to start down earlier.
  • Inoperative equipment (i.e. spoilers, etc.) may keep you from descending as fast so you may need to start down earlier.

As far as the configuration of the aircraft, in order to get the most fuel efficient profile, I would keep it clean as long as possible and start the descent at a point where I could glide for the majority of the descent and configure on speed for a stabilized landing in the normal landing configuration for the aircraft. If this is your goal, then the use of spoilers is an indication of poor planning! ;-)

share|improve this answer

I don't generally plan for long straight-in approaches & descents as I rarely have occasion to do them, but barring any other restrictions the rule of thumb I use for VFR operations in an unpressurized light GA aircraft is "Begin a cruise descent, at no more than 500 feet per minute, such that I arrive at the airport traffic pattern entry at pattern altitude."

You could easily adjust that for a long straight-in to say "…such that I intercept the glide path to the runway somewhere between 3 and 5 miles out without requiring major changes to the aircraft configuration (particularly power/pitch).", or for an instrument approach to say "…such that I arrive at the appropriate fix at the desired altitude, per the approach plate".
I imagine a similar rule of thumb could be established for pressurized/jet aircraft, possibly with a faster descent rate (since you don't have to worry about ear block on the way down).

This indirectly accounts for how much potential energy (altitude) you have to dissipate - you're considering your (ground)speed, starting/ending altitude, and the descent rate you want to maintain (which gives you the distance at which you want to start descending).
The actual descent profile (and your speed profile during the descent) will of course also be modified by the speed and configuration you want when you reach the bottom of the descent - it probably isn't ideal to arrive on a 3 mile final doing 200kts over the ground unless a drag chute is part of your braking routine. (The deceleration requirement is accounted for in the ATC planning formula DeltaLima describes which is also a good starting point for figuring out your descent - for light GA folks like me though you can probably work the speed profile in your head.)


I can't really offer you much for the second half of your question ("How close can you get?") -- That really depends on how high you are starting out (what's your cruise altitude?), the performance characteristics of the airplane (how quickly can you drop altitude?), and how aggressive you want to be as the pilot.

I could conceivably get pretty close in my Cherokee at 5000ish feet AGL flying "straight in", pull the throttle to idle, slip like my life depended on it, and make the runway without carrying too much excess velocity, but it would be particularly ugly to do so, and quite a bit of work for me at the controls.
I would certainly put that in the category of "abnormal operations" (though I also put that kind of stuff in the "fun to try at a quiet airport with my CFI riding shotgun" category).

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.