View Full Version : descent rates from altitude

21st Mar 2002, 01:19
Can anyone remind my tired brain why an jet aircraft descending from altitude will have a greater ROD when the ambient temp is cold than when the ambient temp is warm?. .Lift is a function of air density among other things so this is opposite to what you'd think.. .Any ideas?

21st Mar 2002, 14:50
I think it may be because the descent is normaly carried out at an approx 3 degree angle( for light A/C such as CRJ which I fly)and for a given Mach number the lower the temp the higher the TAS and therefore the higher the rate of descent required to maintain the 3 degree angle.. .If you are talking heavy jets using idle power and FMS calculated decents I am out of my depth. <img border="0" title="" alt="[Cool]" src="cool.gif" /> <img border="0" title="" alt="[Cool]" src="cool.gif" />

22nd Mar 2002, 15:42
I thought about this again and came up with this as a possible..... .At lower temps a pressure alt of say 40 000ft is physically closer to surface than at warmer temp (because warm air is less dense and therefore pressure drop with height will take place more gradually).. .1. The A/C at lower temp has less potential energy for a given mass.. .2. The A/C at lower temp will infact have a shallower angle of descent than that calculated using the simple 3Nm per 1000ft ball park 3degree calculation.. .How this affects vertical speed would I think depend on what you do with the thrust levers(I mean delay descent or keep more thrust on during descent).. .Is this any help or am I just rambling to my self? Or talking Bull....!

22nd Mar 2002, 16:51
Just thinking about this one logically.... .. ....an aircraft is flying along the glideslope - let's use the 3NM/1000' ballpark, and say that our aircraft is 3NM out, and 1000' high. This 1000' will be the physical hight above the ground, not the indicated height, because it is based on the ILS.. .. .In ISA temperatures, it would also be 1000' indicated - the altimeter is calibrated to read the correct height above the datum in ISA conditions. But in colder than ISA temperaturs, it would be slightly higher than 1000' indicated. Let's say it's ISA-20 degrees, the indicated altitude at 1000' would be 1080'.. .. .But in either case, the aircraft has to lose however much indicated height it has within the next 3NM.. .. .Say the aircraft is doing 120kts, it will have to lose this height within 1.5 minutes. At ISA, it has to lose 1000' indicated in 1.5 mins, 667fpm. If it's ISA-20, it has to lose 1080' indicated, giving in indicated rate of descent of 720fpm (although the actual rate of descent is still 667fpm).. .. .The trick is to understand that, in colder weather, the same actual altitude (or height) gives a higher indicated altitude (or height). And that, when we talk about rate of descent, we're talking about rate of change of indicated altitude.. .. .I'm not an airline pilot, though, so don't take this as gospel!. .. .FFF. .---------

26th Mar 2002, 13:29
Bakedbean, . .. .I'm just curious why you believe that a cold aircraft has less potential energy at a given height than a warm one? Potential energy has nothing to do with temperature (at least in this instance). Potential energy is a function of mass, gravity and height. I could be wrong as I am not a physisist, but from what I've learned in physics classes, I think I am right. There may be more complex formulas that provide a better model of potential energy, but I seriously doubt that temperature would be included in it.. .. .E_p = mass * gravity * height

26th Mar 2002, 14:59
FFF,. .. .you say you are not an airline pilot, i sure hope that when you fly those pipers of yours, you don't go anywhere cold. Your story about true altitude is incorrect, the opposite is true. . .When the temperature drops, the density increases. This means that when you fly at 1000' from a warm area into a cold area the volume of air below you will shrink. In order to maintain that 1000' on your altimeter you will have to descend. Therefore your true altitude will decrease and you will have to correct for it if your flying in very cold weather. From -15C to -30C increase your MOCA/MORA by 10%. Ever heard of:"From high to low, look out below"? This is about density, and of you know density is temperature and pressure related.. .Happy landings. <img border="0" title="" alt="[Smile]" src="smile.gif" />. . . . <small>[ 26 March 2002, 10:00: Message edited by: N4641P ]</small>

Hotel Charlie
26th Mar 2002, 15:16
N4641P. .. .Dude! If you read FFF post again you will see he is saying exactly the same as you are!!

26th Mar 2002, 19:28
Hotel Charlie,. .. .You're right, I am saying the same as N4641P - I'm just expressing it the opposite way to him (and the opposite way to the way it's usually expressed).. .. .I said: "in colder weather, the same actual altitude gives a higher indicated altitude.". .. .It's more usual to see this written as "in colder weather, the same indicated altitude gives a lower actual altitude.". .. .I suspect that it's because I reversed this statement from its more common form that N4641P thought I'd got it the wrong way around on his first reading.. .. .I'd still like to hear from someone who knows whether my explanation is correct or not!. .. .FFF. .---------

27th Mar 2002, 14:50
Reading FFF his post another two or three times it does appear that we are saying the same thing in a different way. Getting me all confused and mixed up talking backwards <img border="0" title="" alt="[Roll Eyes]" src="rolleyes.gif" /> .. .Nevertheless I think we can agree on the fact that we mean the same thing.. .Greetings.

27th Mar 2002, 21:38
Hi Phantomwray, sorry I didn't reply sooner I was away. Yeah I guess I just thought my point on less potential energy on a cold day was obvious but I should have said it is because the A/C is physically closer to the ground.In the event of a flight idle descent being used you have less potential energy to trade for kinetic energy to achieve the planned flight path(I think).Sorry for the confusion but I'm sure you weren't losing any sleep. <img border="0" title="" alt="[Smile]" src="smile.gif" /> <img border="0" title="" alt="[Smile]" src="smile.gif" /> <img border="0" title="" alt="[Smile]" src="smile.gif" />

28th Mar 2002, 03:19
Bet you wished I had never started this one! My question was about a flight-idle, fixed IAS descent from altitude, (not maintaining a 3 degree glideslope or true/indicated altitudes etc.). .Flight idle will give more thrust in lower ambient temps, and the wing will give more lift in denser, colder air - so why the greater rate of descent at a given IAS?? Beats me - any more ideas?

28th Mar 2002, 06:30
"My question was about a flight-idle, fixed IAS descent from altitude, (not maintaining a 3 degree glideslope or true/indicated altitudes etc.)". .. .Doesn't colder, denser air create more drag....and the more drag you have (with engine thrust being equal), the faster you descend?. .. ."Flight idle will give more thrust in lower ambient temps,....". .. .Not sure what you mean by "Flight Idle" thrust. Is this what your particular airplane produces when your Anti-Ice is switched on, or your flaps are in landing range? (i.e. high idle). Or do you mean normal idle in the first part of your descent from high altitude cruise? Or does your aircraft have different logic for idle?. .. .I've seen pics of cockpit engine displays during descent where the EPR guages are showing less than 1.0 during descent (i.e. aircraft engines with large intakes/fans at the front). I'm not an engine expert/aerodynamicist, so I'm guessing that a less than 1.0 value equals "drag" rather than "thrust"? Perhaps the colder, denser air equals more drag theory comes in to play here, as well?. .. .Rgds.. .Q.

28th Mar 2002, 15:44
Anybody got access to descent performance data for a heavy may be able to confirm that this is infact the case.

29th Mar 2002, 03:53
I'll buy that. The difference in TAS at FL310 at 300 KIAS in descent in ISA and ISA-10 is only 15 kts, too little to make a big difference to a descent rate. Likewise probably not that much effect on flight idle thrust either. I suspect its the denser air that increases the descent rate ,as you say.. .Thanks gentlemen, g'nite

Ignition Override
2nd Apr 2002, 05:24
Interesting, but does much of this info help a pilot decide (in older planes without VNAV) when to leave FL 330 in order to cross a fix at 12,000' which is ten miles east of Holly Springs VOR?

3rd Apr 2002, 10:51
V.Interesting thread folks.
Perhaps without giving too much away, (if anonymity is your thing) could you give more details of hardware flown to give figures to your premise?
For my sins I fly a 737-300 and am obviously far less observant than I should be as I can't say that I have ever noticed a "marked" difference in R.O.D.
Keep up the good work.;)

4th Apr 2002, 03:02
Well from what i have learned from my instructor , if u need to reach a particular point at a certain height ( with regards to jet aircraft ) , then " Descent point at which the descent must be initiated is three times the altitude " , meaning from FL400 to ground level , u would need to start your descent at 40*3=120 nm out and rod should be 1000 fpm or greater.It works like a charm for my beechjet.I have found this to be the most simple way of flying the descent.

Skyking :)