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When should i descend?

Controlling your aircraft, using the autopilot etc.

Re: When should i descend?

Postby legoboyvdlp » Thu Aug 01, 2019 9:23 am

The calculated fpm (unless you use the aerotexas calculator) will change with true airspeed, which reduces with altitude. That might cause it.
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Re: When should i descend?

Postby Robertfm » Thu Aug 01, 2019 9:55 am

So if I put the aerotexas FPM in the pitch box of course as a minus figure and get the distance from runway right. It should work. I'll try.
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Re: When should i descend?

Postby wkitty42 » Thu Aug 01, 2019 11:57 am

if you can provide an example, maybe we can work through the math? there may be a rounding error which could lead to over/under shooting the target touchdown point...

have you tried turning on the visual glide slopes? i think it is in the view menu in the sim... i think this is the property if you want to set it in the additional settings box...
Code: Select all
# turn on the glide slope tunnels
--prop:/sim/rendering/glide-slope-tunnel=true

"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."
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Re: When should i descend?

Postby wkitty42 » Thu Aug 01, 2019 12:00 pm

i would have thought that using ground speed would have been the thing... especially since airspeed changes with altitude and with head/tail winds...
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."
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Re: When should i descend?

Postby Robertfm » Thu Aug 01, 2019 1:09 pm

Looking at both airspeed and ground speed there seems little difference at lower altitude. Currently flying at 3200ft. Airspeed 150knots, ground speed just under 160. Of course when on approach AS is 100 knots GS 102/3.
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Re: When should i descend?

Postby Robertfm » Thu Aug 01, 2019 1:16 pm

Glide slop can be Switched off & on in . This is my last landing. Approach @ 10nm, speed 100knots, 3000ft , 500fpm to achieve 100ft at runway approach. I used the calculator provided. I had to throttle back about 2 miles out due to excessive descent.
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Re: When should i descend?

Postby wkitty42 » Thu Aug 01, 2019 4:22 pm

the reason i mentioned the glide slopes is to give you a 3 degree approach that you can follow and see if your calculations were even close :)
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."
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Re: When should i descend?

Postby Robertfm » Thu Aug 01, 2019 6:02 pm

I have used it previously but am trying to fly more realistically but I am still finding I am having to make late adjustments in my descent. I am still managing to land OK.
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Re: When should i descend?

Postby wkitty42 » Fri Aug 02, 2019 5:22 pm

right... again, only as a check... so set your formula and when you get to that point, check the slope and turn it back off if you like and let your descent proceed... my thinking was to see if your calculations and settings followed the slope... nothing more... granted, the slope is also limited to 3 degrees when one might want/need more or less slope...
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."
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Re: When should i descend?

Postby Robertfm » Fri Aug 02, 2019 6:28 pm

I will give it a go but I think it only works on Airports that have a localiser and flying with AP. I just used the calculator to land at an airport but it was obvious it was far to quick so I halved the descent and it was almost perfect. No idea what this tells me. As aerotexas is supposed to work I must be doing something wrong.
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Re: When should i descend?

Postby Robertfm » Fri Aug 02, 2019 8:18 pm

Tried another method supposedly used by real pilots: x altitude number by 3 ie 4000 = 12 and that is start of descent in NM. Divide ground speed by 2 & add zero say 100knots =500fpm . Did it work . Nope on a 4000ft approach I was still at 1700ft with the runway beneath me. Even I can't get the calculation wrong.
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Re: When should i descend?

Postby tdammers » Wed Aug 07, 2019 6:44 pm

One thing nobody has mentioned explicitly: IRL, you practically never calculate your descent all the way to the touchdown point. In an airliner, which is where descent calculations matter the most, you will practically always fly IFR, and use published procedures for the arrival. You plan your descent not to the touchdown point, but to the start of the procedure, which will typically specify an altitude to fly.

For example, look at this chart: https://nav.vatsim-germany.org/files/ed ... H_EAST.pdf - it says to cross the ASPAT fix between FL240 and FL280, so that's what you aim for in your descent calculations. Subsequent fixes on the STAR specify further level restrictions, and you need to plan your descent between those to comply with them.

After the STAR, you either fly a transition, or a published approach - in both cases though, similar altitude specifications are provided. For the final approach, a more specific altitude profile will also be provided - the usual procedure is to maintain an intercept altitude (typically something between 2000-3000 ft AGL) until intercepting the glideslope.

As for the descent calculation: I like to use "operational descent" mode for this (it's called "FLCH", "Flight Level CHange" in the Boeings, "OP DESC" in the Airbus IIRC - but it's the same thing): this is essentially a mode where the autopilot maintains airspeed with pitch, and the throttle is set to idle (either manually, like in the CRJ700 which I fly most, or automatically by the autothrottle). This produces the maximum descent rate in clean configuration and at the selected airspeed. In the CRJ at least, if you reduce the selected airspeed while in op descent mode, it will fly level to bleed off speed before continuing the descent, which is exactly what you want, usually. I'm sure the Boeings and Airbuses can do something similar (though since they have full autothrottle, it's probably even easier). For the actual descent calculation, I use a little spreadsheet based on actual performance of the simulated aircraft. It's not perfect, but the basic idea is this: first, we estimate the expected average ground speed through our descent. At a constant airspeed of, say, 280 KIAS, it's going to be somewhere south of 400 kts at cruise altitude, let's call it 420, and maybe about 300 KIAS at FL100; so a crude estimate would be 360 kts. Then we'll take the descent rate we can achieve at 280 KIAS (off the top of my head, about 2000 fpm); divide the total descent (e.g. FL300 cruise - FL100 target = 20,000 ft) by that number (in this example, this gives us 10 minutes of descent). 360 kts is 360 nautical miles per hour, or 6 nmi per minute. So 10 minutes of descent at 6 miles per minute equals 60 nmi, and we'll initiate our descent 60 miles out from the target fix.

This is a fairly good conservative estimate that gets me down comfortably in due time, and doesn't require speedbrakes, which is nice, because speedbrakes basically cost more fuel (you could have initiated the descent earlier, which means you have been burning too much fuel by staying at cruise altitude too long). I basically reserve speedbrakes for when ATC assigns a descent that I can't make without, doesn't clear me for the descent in time, has me fly fast too long (the CRJ doesn't slow down easily), or when it turns out I misestimated my descent calculations.
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Re: When should i descend?

Postby Robertfm » Wed Aug 07, 2019 6:59 pm

Whilst I appreciate the full explanation I fly GA not big craft and VFR, the numbers are a lot smaller but the various methods people have explained and those I have gleaned online sometimes come close other times hundreds of feet too high or low. I use the PHi made to measure distance and I am assuming as it has a scale bar it is designed to be accurate. On that basis and assuming my speed is accurate then it should work every time. There are lots of airports in the UK where there is no ILS glideslope on the PHi map so approaches are already more difficult.
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Re: When should i descend?

Postby SurferTim » Wed Aug 07, 2019 7:26 pm

I guess I learned from the pros back in 1969. It's a math thing. How fast am I going (miles per minute) How far do I have? How much do I need to descend (in feet per minute) ?

If you are flying IFR, ATC will tell you when to descend.
Fly low, fly slow, land on a dime. Twin Otter.
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Re: When should i descend?

Postby Robertfm » Wed Aug 07, 2019 7:39 pm

Most of my approach and landings today have been very close to perfect using the calculator posted earlier. In fact my return to my home airport was probably as perfect as it gets Papi lights 2 red 2 white from 4 miles out. I guess the important bit is measuring the distance to start descent that I put in the calculator.
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