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Consider a small, one- or two-seater motor GA aircraft, something like a Cessna 152 or similar, designed for level flight at roughly neutral pitch.

It's certainly possible to fly such an aircraft in a nose-high attitude while maintaining altitude, by careful use of pitch and thrust. I'm not sure I'd want to do that for any significant length of time, but as long as the angle of attack is kept below the critical angle for the current aircraft configuration, I can see no aerodynamic reason why it can't be done even in the general case.

But is it possible to fly it in a significant nose-down (say, several degrees or more) attitude while maintaining altitude?

My gut feeling says no; no matter what else you do with the aircraft in such a nose-down attitude, you're going to be losing altitude. But I've been surprised before, so: is there a way?

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    $\begingroup$ well - gliders do that while flying in the thermals :) $\endgroup$ – gusto2 Dec 13 '17 at 13:05
  • $\begingroup$ @gusto2 you should make that comment into an answer $\endgroup$ – Dan Pichelman Dec 13 '17 at 17:23
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    $\begingroup$ @gusto2: It can also be done in a light plane if you happen to get into an area of wave lift. $\endgroup$ – jamesqf Dec 13 '17 at 18:47
  • $\begingroup$ Similar to what @gusto2 said, I recall being on a student solo cross country flight in a C152 and entering a significant thermal. I had to reduce power and pitch down a bit below the horizon just to maintain altitude until I exited the thermal. $\endgroup$ – Fred Larson Dec 13 '17 at 19:38
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    $\begingroup$ Although hardly a small aircraft, you might find it interesting that a B-52 routinely climbs (e.g., immediately after take-off) with a visibly nose-down attitude. $\endgroup$ – Jerry Coffin Dec 13 '17 at 22:32
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It depends on the angle of the wing chord line relative to the fuselage and also on the current vertical component of the wind. A normal light GA airplane will not be able to fly level with a negative angle of attack, but it can fly level with a nose-down attitude if the wings themselves are pitched upwards relative to the aircraft fuselage and/or if the mass of air through which the aircraft is flying is rising.

As user2209250's answer alludes to, dropping the flaps is one way to increase the pitch of the wing chord line relative to the fuselage, possibly allowing the wings to have a positive AoA even while flying level with a nose-down attitude. Especially for larger flap extensions, many airplanes will likely require a nose-down pitch in order to maintain level flight with a full thrust setting.

Of course, most airplanes are designed such that you will not be able to maintain a positive angle of attack with a very steep nose-down attitude in level flight in a non-rising mass of air, but shallow nose-down attitudes are certainly possible.

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I may have misunderstood the question but I think the answer may be... flaps. Wing flaps will increase the lifting ability of the wing and at a constant airspeed the nose will need to be lowered to maintain a particular altitude.

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Beyond a very small angle, a nose-down attitude means a negative AoA, and lift will be negative.

A nose-high attitude is different, because lift will be always positive, even beyond the critical stall angle, and for some airfoils, that lift may be quite significant...

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As long as the wing (the whole aircraft) has lift, theoretically, with higher speeds the pitch should go lower and lower.

This means with full thrust, keeping the altitude constant, time will show how nose-down the aircraft can go.

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