Jet climb rates


#1

Looking at flightaware.com/live/flight/FLG2 … N/tracklog

Does altitude determine climb rates assuming no ATC instructions for jets?

Is the thinner air upstairs reducing the climb rate causing this flight to only go 500 fpm at the end of it’s ascent? Looks like autopilot doing a fine job as I very rarely maintain such a consistent climb like the track above.

I generally climb 500 feet per minute on a standard climb in my Sundowner, I think jets go 2000 feet per minute on the initial climb but was wondering why the reduced climb rate?

Allen


#2

Typically in a jet (assumeing to ATC restriction) we climb at an airspeed and take the rate we get. In the XL, for example, there are several climb schedules. I use 260KIAS until .62Mach and then hold .62 to altitude. Depending on weight I can get 4000-5000 fpm initially and end up with 500 fpm at FL410. In the AFM or Ops Manuel you can find time to climb charts that will tell you the same thing they do in the Sundowner, time, dist, fuel and even if a step climb is required. In a turbo fan you actually hurt yourself trying to climb at to high of an AOA. You need to have air moving through the engine to produce power. It’s something that has to been seen to really understand. Also if you get to slow in the climb you’ll get to altitude and never be able to speed up, you’re behind the power curve (so-to-speak).

I remember jump seating on a 737 years ago and as we climbed through FL350 we were only doing 400fpm which shocked me.


#3

Very interesting Leardvr.

Is this because of the thinness of the air, that you suffer climb performance just like us pistons? You don’t adjust engine settings like mixture (I don’t you don’t have mixture control so to speak) but something similar to milk out any additional performance at the higher FLs?.

I guess I can see what you mean for engine performance, as there isn’t enough air density to keep things moving through the engine, so a higher AOA would be detrimental and since the air is thinner, you would never get a chance to get up to speed :slight_smile:

Do you have to advise ATC when getting less then steller climb rates? I was told (hopefully correctly) that if I couldn’t maintain a minimum of 500 during my climb that I was to advise ATC. I have been very fortunate, with me and a passenger, so far I have been able to clime to a big ole FL10 and maintain a 500 fpm climb. I didn’t go higher since my passenger was getting a lil dizzy, so took it back down :smiley:

Allen


#4

Yes, decreased air density affects turbine engines just like it does on pistons.
Interestingly, though, is the differences between Equivilent Shaft Horsepower (ESHP), flat-rated shaft horsepower, and thermodynamic horsepower (THP). (we’re talking about turboprops here. Jets use “thrust” not horsepower.)
Thermodynamic horsepower is the horsepower and engine is theoretically capable of producing. Shaft Horsepower (SHP) is a calculation of the prop rpm and the torque applied to turn it.
For instance, the engines on the airplane I fly have the capability of producing 1450 maximum thermodynamic horsepower. They are flat-rated to only 850 shaft horsepower. This flat-rating allows the engines to operate at much cooler temperatures than the are capable of and also lets it maintain 850HP all the way up to ~25,000ft. Non-flat rated engines start loosing power right after takeoff, just like pistons. The last term is equivilent SHP which takes SHP and includes the thrust that the exhaust produces. The exhaust from turboprop engines escapes at high velocity, just like a turboshaft/turbojet, and can provide 10% or more of the total thrust.

Yes, you’re techically supposed to advise ATC if you cannot maintain 500fpm; though very few people do unless it’s a particularly extreme case.


#5

I also recall that you are supposed to maintain 500 fpm for the final 500 feet of climb to an assigned IFR altitude, but don’t think is often observed either.


#6

In RVSM airspace (FL290-410 in the US) you are supposed to reduce the rate to less than 1000 FPM for the last thousand feet so you don’t cause a false TCAS alert if you happen to be leveling off near another aircraft.

Turbine engines do indeed suffer loss of thrust as altitude increases just the same way a piston engine does. I recall reading once that a version of the Garret TFE731 engine which produces 4300 lbs of thrust at sea level only produces around 1000 lbs of thrust at the max altitude for the airframe it was mounted on.

Can’t speak fo all jet aircraft but all the types I have flown climb well through about FL250 then the rate starts to fall off until I transition to mach around FL290 then the climb rate takes off again.


#7

Sounds incredibly efficient considering the weight the engine has to thrust. It’s not like you lost that much weight in fuel burn on the climb.

Technology at it’s best!

Allen


#8

Assuming the engine manufacturer isn’t derating at low altitudes (like with James’ flat-rated engines), power or thrust from a turbine is proportional to the atmospheric density.

53% at 20,000 ft, 24% at 40,000 ft, 9.5% at 60,000 ft, etc.


#9

Soooo, if I understand this correctly, lest thrust needed at higher altitudes.

Almost sounds backwards in my simple mind.

I figure more juice (thrust) would needed to account for the less dense air to hold up all that weight?

Of course, less friction in less dense air a possibility?


#10

You’re misunderstanding Allen. Mark’s percentages are the thrust available, not thrust required.
You do indeed need more power at higher, less dense altitudes.


#11

Still even more confusing to me I guess…

If at 40,000 feet, you only have 24 percent thrust available, what keeps the plane aloft?

Or is this 24 percent “additional thrust” above and beyond cruise power settings?

Knowing my simple mind, I am probably making this much more complicated then it really is.

Allen


#12

Same thing as your sundowner…
Your engine has 180HP. Let’s say (I have no idea) that it takes 80HP to keep the plane aloft at 1000ft, leaving 100HP extra for climb.
Let’s also say (again, making up numbers here) that at 10,000 ft, you only have 50% of your total power. 60% of 180HP is 108HP, so you’re only left with 28 extra horsepower. Your wing is much less efficient at that altitude also, so it takes more power just to keep the airplane flying. That’s why your climb rate drops, you only have a couple extra horsepower to climb.
Works the same way with jets, only the numbers are way different.


#13

My understanding is that once you get to altitude, you don’t need as much thrust. Something about Newton (?): an object will remain in motion until acted upon by another object. In other words, once you get going and are leveled out, all you need is enough thrust to keep the aircraft in flight.


#14

Well… yeah.
It takes less thrust to stay level than to climb… I don’t know what else you’re saying Dave.


#15

OK, now we getting some where! Makes much more sense this way :smiley:

Thanks all.

Allen


#16

While we’re kinda on the subject, and I have a few minutes, I figured I’d explain why a twin engine airplane looses much more than half of it’s performance with an engine failed.
It can be explained using the same principle as above.
Let’s go with a light twin, like a piper seneca.
Each engine is 200 horsepower for a total of 400HP. It probably takes about 120HP just to keep the airplane flying, leaving you with 280 horsepower for climb. Fantastic.
Now loose an engine. Half as much thrust (HP) but the performance is very severely degraded. Same 120HP to keep the airplane flying (probably more, now that the plane is flying uncoordinated and all.) But you only have one 200 horsepower engine left over, so only an extra 80HP.
Both engines, 280HP extra.
One engine, 80HP extra. 50% of the power, 28% (or worse) of the performance.


#17

So are you training for your new ride? Have they gotten to the part about flying in reverse yet?


#18

I’ll officially be an employee at midnight Friday, as long as FlightSafety gives me the green light. :slight_smile:
First training flight in the airplane is Saturday. http://i47.photobucket.com/albums/f177/cfijames/thumbup.gif


#19

Very cool, congrats! Keep us posted.


#20

I still like the comparison of a boat… It takes more power to get the boat on a plane than to keep it there. The reason to make a step climb or shallower climb is to keep the airspeed up ( stay on a plane) usually with full fuel or a weaker plane (like Cit II). The engines are more efficient at the higher altitudes though so the sooner and longer they get there, the happier they are!!