Why do airliners pitch up during cruise?

Henning Makholm
  • Why do airliners pitch up during cruise? Henning Makholm

    In my experience as a passenger, when the plane stands at the airport and you enter it, the aisle is pretty much horizontal. (Obviously, I've never flown on a DC-3). After takeoff we pitch sharply upwards (duh!), but even after the captain comes on the PA with "we have now reached our cruise altitude", the aisle continues to point a few degrees upward. Usually it's only at "we're starting our descent into XYZ" that the body returns to a more or less horizontal pitch.

    Why is this? I have some hypotheses, but don't know which, if any, of them hold truth.

    1. It's just how the aerodynamics happen to work out. (Unlikely -- it would seem to be a simple engineering matter to attach the wings to the body at an angle such that the right angle of attack for cruise flight corresponded to the body being horizontal).

    2. Pitching the body upwards slightly allows it to generate some lift, free of a proportional increase in drag relative to pushing the cylindrical body straight through the air.

    3. Passenger jets are deliberately designed such that they never need to pitch lower than horizontal in a routine descent, lest passengers might panic and think they're nosediving to their death. (Also unlikely, unless there is really no hidden costs in terms of drag, etc.)

    4. Landings might be more difficult if the plane had to point its nose downwards in order to get closer to the ground -- the nosewheel would hit the runway first. (On the other hand, simply leveling out would count as a flare, so it's not clear to me that's actually a disadvantage).

  • Passenger comfort is a large consideration! It's simply more comfortable for passengers to be reclined by a degree or two than to sit exactly level.

    I have hopped off a 777 onto a small a/c (if I recall correctly, a Baron of some sort) to island hop and the fuselage was pretty much level. It certainly felt less comfortable and somehow, even as if it was fighting not to descend. I would prefer to be a little inclined.

    I believe that some aircraft cruise slightly nose down.

    [EDIT]

    CAVEAT. I am not an expert in physiology or aircraft design.

    Comfort is a big factor and the question is not "do airliners fly pitch up" but "why do they?" That they do is a fact. I suspect that there are many factors, e.g. the fuselage itself, when pitched up, will generate some lift but ask any experienced flyer about sitting in a rearwards facing seat, or walking towards the front when in a descent which results in a level pitch or pitch down attitude. It feels less comfortable than when nose up.

    If the aircraft is level, and you are level, you experience +1g vertically and 0g horizontally. If you recline the seat, you still experience +1 and 0. If the aircraft is pitch up, you experience a small -ve g component in the horizontal. Quite what effect this has psychologically, I don't know but it does make a difference. I've heard, anecdotally, that facing backwards, e.g. in a British Airways biz class seat is less comfortable than in a forward facing one for exactly this reason.

  • You need a slight pitch-up attitude to fly: you need a certain angle of attack in order to produce the required lift to remain airborne.

    As you have imagined, usually the wings are tilted with respect to the fuselage so that in cruise the wings will have the required AoA while the fuselage will be horizontal to minimize drag (and thus fuel consumption).

    Passenger comfort is much less important than this (and passengers can always lean back their seats).

    My hypothesis for your observation is that the "pitch-up" you observe is either a visual/sensory illusion due to the lack of external point of reference (no, the clouds are not a valid reference and the ground is too far away) or you have flown in aircrafts where the floor is not parallel to the aircraft x-y plane. Personally, I would lean towards the first hypothesis, as having a non-horizontal floor in flight creates all kind of nuisances to cabin service (again, something you want to avoid).

  • Ultimately the answer to your question is number (1) from your options above - That's just how the aerodynamics happen to work out.

    To generate a certain amount of lift requires that the wing have a specific angle of attack at a given speed. As the wings are more-or-less permanently attached to the fuselage setting the angle of attack requires us to pitch the entire aircraft, changing the deck angle.

    There is a very nice web page illustrating this in (greatly exaggerated) detail, so I won't reproduce the full explanation & drawings here.


    As to why the aerodynamics work out that way - that probably has something to do with numbers (3) and (4) - passenger comfort, and ease of executing maneuvers (like landings).

    Most airliners I've been on have a slight nose-up deck angle (maybe 2 degrees) in cruise, which isn't really "noticeable" to most people unless you're carrying a bubble-level or staring very intently at your drink, but the exact angle for climb, cruise, and descent will vary depending on the aircraft and how it's been loaded (you can pretty much descend with any deck angle you like).
    Most people would probably object to flying around with a 10 or 15 degree deck angle (whether nose-up or nose-down), so as a concession to passenger comfort the wings are attached to the fuselage in a way that is aerodynamically favorable (not trying to drag the fat bottom-side of the plane through the sky) and comfortable for the passengers.

    Conversely most GA aircraft I've been in seem to cruise with a flat or slightly nose-down deck angle, though that could just as easily be an optical illusion from having the big window in front where you can see it. (I'm estimating the deck angle based on the wet compass tilt since I generally don't serve drinks.)

  • Mainly the options 2 and 1 from your list.

    What nobody mentioned so far, but is rather important here is that the angle of attack depends on speed1 and weight. While the "speed" does not vary much for most airliners, the difference between empty and fully loaded airliner is rather large.

    Now if the fuselage was tilted down, it would generate lift directed down and associated drag with the net result of simply a lot of extra drag. So the aircraft is designed so that it does not fly (in level cruise) pitched down even when light. But that means that when it is fully loaded, it flies slightly pitched up, because it needs more lift and higher angle of attack to get it.

    1 It depends on the "indicated speed", which is not speed at all but rather dynamic pressure expressed as speed at which it would occur when moving through air of standard sea level pressure (1013.25 hPa). The pressure at high altitudes where airliners normally fly is significantly lower, so the indicated speed is usually does not exceed 250 knots when true speed is well over 400 knots.

  • I believe the answer may be option 5:

    5. The aircraft changes during flight.

    At the beginning of a flight an aircraft is loaded with fuel. This increases the weight of the aircraft and thus requires an increase in the AoA to maintain level flight. As the aircraft burns fuel the mass of the aircraft decreases and consequently the AoA must decrease also.

    Depending on the aircraft type and the length of the flight, the weight of the fuel can be very significant. For example, a Boeing 777-200 has a maximum fuel capacity of 117,340 lts. This ammount of fuel weighs aprox. 95,000 kg. The maximum take off weight for this type of aircraft is slightly over 247,000 kg. This means that a 777 fully loaded with fuel and with maximum take off weight will lose a bit less than 40% of its mass during flight.

  • You actually have some pretty good hypothesis here!

    To start with, it truly is "a simple engineering matter to attach the wings to the body at an angle such that the right angle of attack for cruise flight corresponded to the body being horizontal", but you missed the design goals slightly. This angle is called the angle of incidence, or the rigging angle of the wing and is the difference between the fuselage angle and the wing angle. This is a very specific angle that is calculated as part of the design of the aircraft.

    angle of incidence example

    The wing incidence must satisfy the following design requirements:

    1. The wing must be able to generate the desired lift coefficient during cruising flight.
    2. The wing must produce minimum drag during cruising flight.
    3. The wing setting angle must be such that the wing angle of attack could be safely varied (in fact increased) during take-off operation.
    4. The wing setting angle must be such that the fuselage generates minimum drag during cruising flight (i.e. the fuselage angle of attack must be zero in cruise).

    These design requirements naturally match with the wing airfoil angle of attack corresponding to the airfoil ideal lift coefficient (see figure 5.26).

    The typical number for wing incidence for majority of aircraft is between 0 to 4 degrees. As a general guidance, the wing setting angle in supersonic fighters, is between 0 to 1 degrees; in GA aircraft, between 2 to 4 degrees; and in jet transport aircraft is between 3 to 5 degrees.

    Notice that the proper angle of incidence will have the wing at its most efficient during cruise while the fuselage also generates the minimum amount of drag. Both of these are calculated independently and the angle of incidence is set so that both conditions are satisfied at the same time.

    Aircraft design decisions are usually a compromise between different, many times conflicting goals. He goes on to say that it may be adjusted from the optimum in some cases:

    The wing setting angle may be modified as the design process progresses. For instance, a fuselage with large unsweep over the rear portion to accept aft cargo doors may have their minimum drag at a small positive angle of attack. In such cases, the wing incidence will be reduced accordingly. Another, les fundamental, consideration is that stopping performance during landing operation to get as much weight on the braked wheels as possible. Thus, there is a benefit to reduce the wing incidence slightly to the extent that the change is not felt significantly in the cabin. Reducing the nose gear length will do the same thing. This technique is limited in passenger aircraft because a level cabin floor is desirable on the ground. But, for fighter aircraft, the level floor is not a design consideration.

    Another possible reason to adjust the angle of incidence to a non-optimum number is to ensure that when landing the aircraft the nose is not pitched down, in order to avoid a higher likelihood of hitting the nosewheel first on landing.


    Most of the material for this answer came from this document written by Mohammad Sadraey at Daniel Webster College

Related questions and answers
  • In my experience as a passenger, when the plane stands at the airport and you enter it, the aisle is pretty much horizontal. (Obviously, I've never flown on a DC-3). After takeoff we pitch sharply upwards (duh!), but even after the captain comes on the PA with "we have now reached our cruise altitude", the aisle continues to point a few degrees upward. Usually it's only at "we're starting our descent into XYZ" that the body returns to a more or less horizontal pitch. Why is this? I have some hypotheses, but don't know which, if any, of them hold truth. It's just how the aerodynamics happen

  • In As the Pro Flies, John R. Hoyt writes (pages 41-42): Suppose we have to land in high, gusty winds. That's what happened to Pilot Z, who once landed his plane during such conditions with his flaps down. After the wheels were on the runway he relaxed, never realizing that a plane is not landed until the switches are cut. Because he still had airspeed and because full flaps lowered the take-off speed, a small gust of wind was all that he needed to begin flying again. The additional lift was enough to raise him 10 feet from the runway, and at that point he ran out of gust

  • that the aircraft went that way. In reality, this would be more complicated. For example, the plane most probably did not go along a straight path at max speed. However, useful inferences can be made by adding other information available. For example, from the ping circles separated by one hour, we can get plausible directions the plane may have taken. Why isn't anyone pursuing this line of analysis... if the plane most likely went along the S or the N arc we see in reports. Unfortunately, only the last ping (at 8:11AM) is available publicly. Here is the basic idea on extracting the information

  • this to happen? (My guess is it is CG related) And most importantly: If I would have continued this "mushing" flight, would it be possible to have entered a flat spin or a simple "drop out of the sky.... This "mushing" went on for what seemed ages before I eventually applied power and pushed the nose down to gain airspeed again. We tried it again after that and the same thing happened. I had an instructor...When I took delivery of a new Cessna 182T last year, I did a test flight for certification purposes. During the test flight we had to perform a power off stall but that didn't go as planned

  • After answering this question on History.SE, I started to wonder if it would be possible to find out even more detail about the plane now that its serial number is known. I have no idea what kind of flight records the US Army Air Corps kept, however. I know most flight logs today are kept by pilot, but I imagine there would be some way to trace what pilots flew a particular plane. I have no idea if this is possible for USAAC trainer planes in the 1930s. Could I get access to these records? If so, how would I go about it? I'm mostly interested in seeing if I can find out more information

  • Here are a few thoughts: 'Real' accidents happen much too seldom to be of any real measure, and they would have to be compensated for the number of passenger kilometers as well to be objective. Large airlines may have be involved in more accidents, but they have more aircraft. Many airlines low down on the reports had accidents many years ago. Avherald and the like may be good sources but emphasize that they don't report on all accidents. Different jurisdictions have different reporting requirements. What is a fair and unbiased method of measuring airline safety?

  • The alpha vane is an external probe used to measure the angle of attack. I have been trying to understand how exactly it works, but I can't find any clear explanation or simulation. Is the vane static or dynamic i.e. does it rotate along its central axis? Given that it has a significant surface area, I think that it would either: Rotate because of the force/drag exerted by the airflow, and give an angle of attack proportional or equal to its angle of rotation Measure the force being exerted on it via a force sensor embedded in the surface Is either of these correct? In short, how

  • I was looking through my virtual radar logs one of the days and found this "glitchy" ADS-B behavior. I am almost 100% sure that this is not due to my antenna or setup since two independent different radars confirmed this weird behavior from FlightRadar24. Also A/C before and after this one did not exhibit this behavior. Does anybody have any thoughts as to what may be happening??? Why... of occurrence is approximately: 3/16/2014 6:09pm CST I have also verified FlightAware is ALSO showing the same weird glitch. See below "yellow" highlighted airplane: Same A/C from FlightRadar24

  • protect the president whilst in the air? I have heard of TFRs for "VIP in the area" reasons — is that for AF1? I am guessing that the aircraft identification is blocked, but wouldn't they still need to have the transponder on for TCAS? Specifically, the Wikipedia page on Air Force One has the following quote: Air traffic controllers gave Air Force One an ominous warning that a passenger jet was close to Air Force One and was unresponsive to calls. "As we got over Gainesville, Fla., we got the word from Jacksonville Center. They said, 'Air Force One you have traffic behind you

  • I hope this is a relevant place for me to ask a math question regarding aircraft design. I am trying to understand how one would implement a controller to control the pitch angle of an airplane... serves as the starting point. What is the starting point or what are the principles used to derive these equations? If I know how to derive these equations for a very simple case, then I know I have... these are longitudinal equations of motion although their general form differ from each other. I think I got to understand one point: these equations were derived considering translation motion on the x and z planes