TOD and descent rate

Hi all,

I have watched the new IF tutorial’s video about descent and also the Captain Joe’s one and I’m still doubting about which one of these videos are the more accurate to the real life.

To simplify, Captain Joe says that the TOD should factor winds (1NM per 10kts wind), the altitude to lose (current FL divided by 3) and speed reduction (1NM per 10kts IAS) by the way, he says the average descent speed should be 200kts IAS and for the descent rate, it’s: Current altitude (in feet) divided by the time of the descent (TOD / 200kts) but it’s not steep enough. So, here’s his descent profile.

However, in IF tutorial’s video, the formula is different, for the TOD, it’s altitude to lose / 1000 x 3 and for the descent rate, it’s GS x 5. In this case, the descent rate doesn’t change (except if you change your speed).

So, which one is the most realistic?

Thank you for answering me.


I recommend just using IF’s built-in VNAV Descent feature - which does all the calculations for you automatically.

In terms of which one is more realistic - not to shame the devs but most likely the IRL one.

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I just use VNAV

However use this math to show off to your friends

I have tried the VNAV feature, but the V/S is not steep enough since I reduce my IAS. I don’t say it doesn’t work, but I’ll have to keep the same speed so?

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Formulas for altitude per NM (-1000 ft every 3nm) and in terms of descend rate formulas - I would rather stick to the IF ones as they are based on VS and generally more precise because, as we know, angle is depended on the intended descend rate and speed, and you get all of these things in IF’s method

IF you descent manually you have to watch your speed,as speed changes so does v/s.

Edit:even if you don’t change your ias, ground speed will decrease as you lose altitude.

Mathematically, Joe’s calculations actually match: 10nm per 3000ft matches 5x GS, 3nm per 1000ft is not enough.

About factoring in wind: that’s actually more relevant on certain aircraft like the 787. In the 787, a 3 degree descent relative to ground in a tailwind will make you speed up. In this case you may need to do 3 degree descent relative to air (so 5x TAS).

The formula in the video is plain wrong actually. 5xGS mathematically requires 10nm every 3000ft, not 3nm every 1000ft. (I know mentour uses the second, but 737 can idle descent faster than 747 I think…)

Wait a minute, how do you descend from FL300 in 11nm in a commercial aircraft?

I just cut throttle and glide at 250 knots. It’s efficient.

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You don’t, the diagram from Joe’s video just means that from cruise to 8 nm out you adopt a 3 + degree descent, the “1000 feet” and “3 nm” are just there for you to make a relation to the last 8 nm to the runway, as it is a lot easier to compare 3 nm, 1000 feet to 5 nm, 1000 feet than it is to compare 84 nm, 28000 feet to 5 nm, 1000 feet.

As I understand, The Captain Joe’s descent rate is mathematically correct but VNAV and IF’s tutorial is simpler?

Finally! Someone else who agrees with me!

I do this:
(Altitude MSL you are at - altitude MSL you want to be at, i.e. a restriction)/1000x3 which is whst they do in the video
I then disengage A/T and cut the thrust lever to idle
I then (and you need to do this quickly) start lowering the V/S. I usually go straight to -3000FPM. You then nedd to adjust the V/S to maintain your desired mach number.
Once you reach FL280, start reducing/increasing (likely increasing) V/S to maintain your desired IAS. When you pass your first restriction, I typically then engage VNAV, as I have not worked out any better ways.

Okay, thank you! I’ll try it and I’ll PM if it works (or doesn’t) P.S.: You’ll not have the message before at least two weeks as I don’t have much time to do a flight

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The reason the VNAV doesn’t work based off your calculations is because your calculations are for determining a 3 degree descent path (or idle descent depending on the aircraft), the VNAV is set to initiate descent from cruise at a 2 degree path and level off at each altitude restriction until the 2 degree path returns (unless the altitude restrictions require greater than 2 degrees between those points)

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