Top of Descent calculation

[a1stogg]

I made an excel file that calculates when you should start your descent in order to maintain a 3:1 glide slope. It's on my OneDrive if you're interested: https://onedrive.live.com/redir?resi...21BC601%212028

Here are the formulas if you would rather run the numbers yourself...
Distance from waypoint = 3 x 'altitude difference' / 1,000
Descent rate = 'altitude difference' / ['distance from waypoint' / 'GROUND speed' x 60]

Example:
Assume you're at 30,000 feet, you want to be at 3,000 feet by the time you reach your next waypoint and you will be descending at 250 Kts (ground speed). Your 'altitude difference' equals 30,000 - 3,000 = 27,000 feet. This is how much you need to descend before your next waypoint. Now you just plug the numbers into the formulas.

Distance from waypoint = 3 x 'altitude difference' / 1,000
= 3 x 27,000 / 1,000
= 81,000 / 1,000
= 81 NMs

Descent rate = 'altitude difference' / ['distance from waypoint' / 'GROUND speed' x 60]
= 27,000 / [81 / 250 x 60]
= 27,000 / [0.324 x 60]
= 27,000 / 19.44
= 1,388.88 fpm

So to reach 3,000 feet by the next waypoint you would slow and maintain 250kts (ground speed) in time to start your descent of 1,400 feet per minute at 81 nautical miles away from that waypoint.

[th3k0p]

This was very helpful.. thanks! :loyal:

[Wingman]

This is definitely one of the most common things to have to figure out on a per-flight basis (unless you're flying a forest height reduction mission).

Other useful rules of thumb (some of which speak to concepts we don't frequently take into consideration when flying in FSX/Flight, but should if we want to be better pilots) can be found here: http://www.planeandpilotmag.com/prof...-of-thumb.html
For example, how often do we really consider density altitude's affect on takeoff distances when trial-and-error works just the same in a simulator?
What about changes in ground speed and their respective impact on landing roll (it's significant!)?

[Storm]

The standard glideslope is 19:1, not 3:1. That gives you the standard 3° descent angle used for ILS. By contrast, 3:1 is an 18° descent angle; both your passengers and your airframe would not thank you for that!

Here's something simpler you can easily do in your head while flying. For a standard 3° descent angle:

• Rate of descent (ft/min) = 5 x ground speed (kts). (Approximately. More accurately it's 5.3.)
  E.g. if your ground speed is 200 kts, you need a descent rate of 1000 ft/min.
• Calculate top of descent by time, not distance.
  Time to start descending to waypoint = altitude difference/rate of descent.
  E.g. if you're descending from 12,000 ft to 2,000 ft at 1000 ft/min, start your descent 10000/1000 = 10 minutes before your waypoint.
  You can find the time to your waypoint in several places, like some DMEs or the bottom right-hand corner of a Garmin 500 GPS.

HOWEVER, this assumes a constant ground speed, which is not going to happen! Your ground speed at high altitude will be far higher than your indicated airspeed. Or, put another way, at altitude your indicated airspeed will be a lot lower than your ground speed. As you descend, the difference between the two diminishes.

Are you really going to keep increasing your airspeed as you descend to maintain a constant ground speed? No! More likely you will maintain airspeed and let the ground speed decay towards it. Which will mean you will arrive at your desired altitude before you had planned and you'll need to fly level for a while. But that's OK, as you need to be below an ILS glideslope, and to fly level into it, in order for your autopilot to establish onto it reliably.

Another similar frequently required estimate is "Where shall I fly onto the feathers?". If (and only if) the ILS uses a 3° glideslope, use the rule-of-thumb of 300 feet/nautical mile. (More accurately this is 318 ft/nmi.)
For example:
Assume runway elevation is 600 ft asl.
Assume you're approaching level at 3000 ft asl.
Difference is 2400 ft.
Therefore intercept the feathers at least 2400/300 = 8 nmi from touchdown.

Another similar question is "What altitude shall I fly onto the feathers?". If (and only if) the ILS uses a 3° glideslope, once again use the rule-of-thumb of 300 feet/nautical mile.
For example:
Assume runway elevation is 600 ft asl.
Assume your intercept is planned at 8 nmi from touchdown.
You need to intercept at no more than 8 x 300 ft = 2400 ft above touchdown elevation.
Therefore fly level at 2400 + 600 = 3000 ft asl to intercept.

[a1stogg]

By 3:1 I was referring to dropping 1,000 ft every 3 NMs. S̶t̶a̶n̶d̶a̶r̶d̶ ̶g̶l̶i̶d̶e̶ ̶s̶l̶o̶p̶e̶ Let's call it normal descent rate instead. I just wanted to share some info I came across, and have found useful, with anyone interested.

I like the idea of using airspeed more than ground speed though, so I'll mess around with my calculations and average it out a bit more rather than just using a standard angle from current altitude to target altitude.

No matter how you determine when and where (or maybe even why) to descend - flying is a fluid thing... winds change and cause your steady IAS to produce differing ground speeds which in turn change your ETA to whatever waypoint you're descending to.

ToD calculations can be used to help you understand the process of descending. The more you use 'em the more you'll get a feel for how the aircraft you're flying will drop out of the sky at a pleasing angle and rate.

[Wingman]

All this math talk is making me think up a challenge: Roughly what would my minimum rate of climb need to be if I have 750 feet of runway + grass left before I have to clear a 50 foot obstacle (tree) in my aircraft that just rotated on takeoff and is doing 90 kias?

Hey, I like the idea of a regular challenge question to maintain or expand our general pilotage knowledge!

[a1stogg]

608 fpm is what I get...

750' = 0.123434125 NMs. You'd travel that far in 0.0822894167 minutes if you were going 90 Kts. 50 feet per 0.0822894167 minutes equals 607.6115496393 fpm.

Check out this video of a short field takeoff... And near miss! :-)
http://youtu.be/ZmEVwyMRYIY

[Storm]

By way of information, the international standard abbreviation for nautical mile(s) is nmi, not nm. By contrast, nm is the international standard abbreviation for nanometers (billionths of a meter).

1 nmi = 1.852 trillion nm
That's quite a scale factor.

"But," I hear you say, "my FSX GPS says nm. Surely this is must be right? We all know Microsoft produces bug-free software, don't we?" Actually Microsoft is just copying what Garmin does - which is also wrong. I guess they were saving on screen real estate.

Digression over. Carry on gentlemen.