Cruising at high altitudes

[Bubblehead]

I'm not a pilot but an avid aircraft enthusiast and a long time flight simmer. I've always liked to learn more about flight. I have a question. When a jet liner reaches their cruising altitude, let's say 37K feet, where of course the air is thinner and thus affect the lift but then the drag decreases, does the pilot adjust air speed to compensate for the variables?

[Speed of flight]

You've bitten off quite a large piece there, but I'll try to hit the highlights, and maybe others will give some more detail.

Every aircraft will have a certain set of limitations, and will become known as it's "flight envelope". This is where a particular aircraft can fly. You may hear terms like "coffin corner", which is a limit in the flight envelope, if one were to draw it on a graph. Hard to explain, but easy to understand with a picture.

Ok, so we've established that aircraft have limits. Usually those are dictated by weight and speed. More variables: the magic 18,000 ft mark, known as FL180. Half of the earths atmosphere is below FL180, so that means that the other half is above, and the atmosphere can be upwards of almost 200 miles out. Answer: yes, density is a factor. So much a factor that at sea level, standard day (59*F, 29.92 in Hg) Mach 1 is 762 MPH (from memory, I think it's within a couple MPH of that) but is only 350kts indicated at what FL350 (somewhere around there). Those are 2 different ways to measure speed, but it illustrated the difference in speeds to achieve Mach. Point: you achieve Mach at a slower speed through the air at altitude than on the ground, and that's because of density. Airliners nowadays (a least most of them) will show very clearly where the envelope is in the Aircraft Flight Manual, and on the MFD, there will be a red tape on the left side of the airspeed indicator showing the Vne (velocity-never exceed) speed for your configuration.

Cruising altitude, weight, temperature, humidity. All major factors in where your airplane will perform on any given day.

Look up the Lockheed U-2. That is one of a few airplanes that had an interesting flight envelope. At its maximum altitude, it reaches a "coffin corner" so tight that it has been said that 1kt slower is its stall speed (where the wings' laminar airflow departs the wings surface ahead of the center of lift, and no longer creates lift) and 1kt faster is breaking Mach (the airplane was not designed to break Mach, and would have broken the airplane also).

To be more direct at your question, the pilot has a number of ways to compensate for air density, gross weight, and temp/humidity. On a humid day, air is physically being displaced by moisture. Less air density, less lift potential, so speed must increase to compensate, or weight must decrease to accommodate.
A guideline my instructor gave me: 4H's-High Humidity, High Density Altitude, High Temperature, HORRIBLE day to fly.
Speed can increase to a limit, but after that, what do you do? To create the same ammount of lift at a slower speed, an aircraft can increase its Angle of Attack, which is the difference between the angle of the wing chord to the relative wind. Picture a surfboard: it's nose is out of the water, while the tail is below. Wings do that. Realistically, though, there is a limit, and you get there long before the physical limit (around 20*) and it's somewhere around the time your stewardesses get run over by their own beverage cart. You can't have your airliner at a 18* AOA for a transoceanic. Not comfortable.
Example: a B777-200LR direct from KSFO->OMDB (Dubai, UAE) makes it's first flight legs at FL290 because it's full of fuel, pax, and simply couldn't keep stable higher than that until some of that fuel burns off. Get it below 80%, and step climb to FL370. Done.

Oh my. I hope I wasn't too wordy! Maybe though, some of those terms can get your dictionary busy and you can start making those long haul great circle routes without cheating.

Hope there was help in there somewhere, and see you in the wild blue!

[Sprocket]

You've bitten off quite a large piece there, but I'll try to hit the highlights, and maybe others will give some more detail.

Wow..super read so far, thanks

[Bubblehead]

Although you failed to make me an expert I've had enough engineering (math, physics, etc.) training to decipher what you were talking about. Lots of variables. Lots of prep. Pilot doesn't just get on the plane and figure out things once he's airborne. Does the autopilot help out do most of these adjustments? Also you missed mentioning flight efficiency, you know - miles per gallon.

[PhantomTweak]

Does the autopilot help out do most of these adjustments?

No, the autopilot is actually, for all the sophistication they try to add, a fairly stupid animal. Most of the sophistication seems to be on the landing end of the AP. Otherwise, it pretty much will keep you pointed on the same heading, or keep the wings dead level, damp out yaw wanderings....and maintain a constant airspeed and altitude. Barometric altitude, normally, being too far up for radalt use. The f-111 and A-10 have excellent Terrain Following ability, based on radar altitude, and can be set for different "hardness" or how tight it follows ground contours. IE: Will it beat you to death with your own harness rising and falling with every gopher mound and fallen log, or give ya a nice steady ride. it's a balance. the staedier the ride, the farther up you have to be to avoid ground contact. The system doesn't look ahead very well, and will fly you into the side of a hill quite happily.
Now, having said that, bear in mind that Indicated Airspeed (IAS) is dependant on altitude, temperature, humidity, and so on and so on. On todays airliners, there is a pretty good Air Data Computer (ADC) that takes most if not all the variables into account. THAT is the speed the AP will maintain, whether as an indicated Mach number (0.83 or whatever) or as IAS. Formerly, back in the 60's to the 90's or so, this was all done mechanically, now it's all digital. Aligning the ADC on an F-4 or a Kfir was an all day task, trust me.

Also you missed mentioning flight efficiency

...

THAT is an ongoing adjustment. As the plane looses weight, by burning fuel, it takes less and less effort to shove it through the sky. Also, Jet engines have a "sweet spot" altitude/airspeed/ambient temperature combo where they get the best fuel efficiency. The ADC can display the fuel consumption per unit time....Gallons Per Hour, Pounds per minute, whatever, and the pilot can then adjust the altitude and airspeed according to predetermined charts in the flight manual to acheive the best possible combinations. As far as I know, even the best ADC/AP combo can't/won't adjust for fuel consumption, altho I've been away from the industry for a while now, and given the prices lately that may have changed.

Don't forget also, the flight crew is bound by other variables, such as ATC assigned altitudes and airspeeds, the jet stream direction and altitude, other traffic, and so on. Unlike a Sim, real airliners can't just pick a direction, altitude, and airspeed and plow on through

I know, I babble endlessly, sorry bout that. Hope this helps a little. It's not easy to discuss/explain such a complex subject in simplistic terms!

Pat☺

[Bubblehead]

I'm listening intently. Thanks.

[C]

Generally (in the airliner altitudes above 28000'ish) you'll specify a cruising Mach no on the flight plan so that is what you'll fly. Up to the point you covert to a mach no, you'll fly a constant IAS. Back on my old type (which was slightly vintage), we'd hold 290KIAS (in the climb) or 300KIAS (cruise) below 33000, then hold M0.84 above, regardless of the altitude.

An aeroplane probably could do things more efficiently, but in busy airspace ATC like you to be predictable, speed wise.

[popejose]

Kind of along the same lines I have a question.

When at high cruise altitude (say FL350 or higher) the aircraft will have a nose up attitude of up to 5%. Is that an issue with shotty flight dynamics or am I not making some adjustment? This has got to be affecting fuel consumption.

It seems that the plane is barely leveled at 330KIAS and sags more and more as you slow.

/regards

[PhantomTweak]

When at high cruise altitude (say FL350 or higher) the aircraft will have a nose up attitude of up to 5%. Is that an issue with shotty flight dynamics or am I not making some adjustment? This has got to be affecting fuel consumption.

No, not shoddy adjustments, nor does it signifigantly decrease efficiency. It hearkens back to the issue of AOA vs airspeed vs flight limitations of the aircraft. I'll try to keep it simple (yeah right, ME????)...

For the sake of this discussion, I am going to presume you know the difference between AOA and angle off the Horizion, or pitch.
Most, if not all, commercial aircraft have Vne (Velocity, never exceed) speeds below Mach 1.0. As explained above, Mach number is used to indicate speeds at high altitudes, as it takes into account all the different variables such as humidity, temperaure, and so on much better than just IAS does. Now, the higher you go, all other things staying equal, which we all know they don't, the slower in Knots or MPH Mach 1.0 gets. IE: 776 MPH at sea level, and let's say for the sake of discussion, 325 MPH at 35,000' MSL. Many reasons for this such as humidity (almost 0% above about 27k.ft), density of the air, etc etc. Having said that, the aircraft's Vne does NOT change with altitude. if it is, say, M0.88 at 0'MSL, it is STILL M0.88 at 35,000'. So, 776x0.88 gives us a VNE at 0'MSL of 682.88, but at 35,000'MSL it is only 286. With me so far?

Please note that ALL numbers used in this discussion are for explanation ONLY and are NOT real life numbers!!

Ok, the aircraft's wing needs, let us say, 300MPH for 0° AoA, level, Constant altitude flight, then it would be exceeding M1.0 at 35,000 MSL.
Since the aircraft's Vne is only M0.88, to try and fly at 0° AoA at altitude would destroy, or at least severly damage, the airaft. Since the Airspeed for level, 0° AoA flight is well above Vne, you must increase the pitch to increase (in this case) the AoA for level flight. So, to maintain constant airspeed and altitude at 35,000'MSL you have to increase the AoA to +5°. In straight and level flight, that also means pitching up 5° above the horizion.

Additionally, as explained in my first post, we know that jet engines have that "sweet spot" altitude/airspeed combo. Once you find this using the aircraft's flight manual charts and a little experimentation, then you must also set you airspeed below Vne, but as close to the engine's best airspeed as possible. It's all a balancing act. As the engines efficiency increases faster by far than the parasitic drag caused by the higher AoA, overall, then no, all this doesn't really decrease the aircraft's overall efficiency.

Totally and completely lost now???

Hope it helps a little bit, anyway

Pat☺

[Speed of flight]

Bear with me, I'm typing this from my phone.

So, for means of discussion, there are a few ways to achieve a lower AOA, if that is your concern. You will alway have something of a positive AOA, provided you're loaded and fueled correctly. However, another "sweet spot" is highly sought after. If you are looking or a maximum fuel efficiency factor, load your aircraft toward the aft CG limit. If it's somewhat tail-heavy (while still observing the aft CG limit, of course) then the elevator doesn't have to work as hard to push the tail down to keep the nose up. It may yield a somewhat uphill AOA, but if you trim correctly, it really shouldn't change anything. By doing this, you've made less "induced drag", lowering fuel consumption, and allowing more payload/fuel.
My instructor taught me that one.

Ok, so if AOA is your issue, then we need to know where the envelope is. If it's excessive or even at the limit, then you need to speed up, or descend. I always leave a little speed cushion for gusty conditions, so maybe 15 kts below max. Now we've nailed down the speed, and can't go faster. Still too positive, well then you're too heavy. If you're full of cargo/pax and fuel, your envelope limit is severely handicapped. Either jettison pax (or maybe cargo instead, your choice), or sink to a lower altitude. Your POH or AFM should have your limits neatly graphed out.

I love planning a long heavy flight in the sim. I usually take the PMDG MD-11F across the Pacific from anywhere west coast USA to Hong Kong. Lots of variables there.

Hope this helps!