What is a rule of thumb for when to begin a descent?

Question

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?

Answer 1

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! ;-)

Answer 2

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.

Answer 3

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.

Answer 4

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).

Answer 5

Say you're cruising at 17000ft(I know this is low) at a speed of 310 knots and landing at a runway 700ft above sea level. This means you need to descend 16300ft.

First off, cross out the last two figures so you're left with the number 163. You then divide that number by 3 to get 54.3. Let's say you're landing at 200 knots so you need to lose 110 knots.

Divide the speed you've got to lose, so 110, by 10 so in this case you're left with 11. Add that to the first answer, 54.3, and then you have the distance out you need to start your descent at, in this case 65.3 nautical miles.

If there is a tailwind then for each 10 knots minus 1 nautical mile and for a headwind add 1 nautical mile for each 10 knots.

For working out what rate you should descend at, divide your total nautical miles figure, in this case 65.3 by 200. You should get what is rounded to 0.33. Then work out what that is as a percentage of 60. 33% of 60 = 19.8. That's how long the descent will take so divide the altitude you need to lose, 16300ft by the time it will take, so 19.8 minutes but say 20, you just round it. 16300/20 = 815.

This means you will start your descent at 65.3 nautical miles out and descend at a rate of 815 feet per minute. As the fpm goes up in hundreds round down to the nearest 100 so in this case 815 becomes 800.