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Can an autopilot stall in Pitch hold mode?

We all know that an autopilot will eventually stall the aircraft in Vertical Speed hold, but is the same true in Pitch hold mode?

My first guess was yes it will stall. But if a constant pitch is held as power decays then climb rate will slowly decay. Eventually, the aircraft will reach equilibrium and stop climbing. If the pitch is below the stall critical angle for level flight then surely the aircraft will never stall?

EGEO

What do you mean by stall critical angle ? Cannot any positive pitch angle be stall critical depending on power and airspeed ?
Apart from specific negative pitch angles at or below best glide, keeping a level or positive pitch angle fixed, while slowly reducing power, will have the aircraft climb, then fly straight, and if you reduce power further, it will stall. Any pitch angle is a stall critical angle depending on the airspeed. No ?

Safe landings !
EDLN, Germany

The angle of attack is the angle between the chord line and relative wind, or the free airstream vector. Once the aircraft starts to descend even holding the chord line parallel with the horizon, the relative airstream is from below, and at some point will exceed the critical angle.

Oxford (EGTK), United Kingdom

While (as you rightly point out) the PIT mode is much better for probing the operating ceiling than the VS mode, I have found that the plane will still stall eventually.

I can’t explain why (not clever enough). I guess because the ceiling is defined as the altitude where you are on the onset of the stall, so it must get to that point eventually as you climb

The IAS-hold mode is better still, if you have it, if the pilot is completely asleep, but you can’t use it to reach the operating ceiling because, obviously, the speed you want to climb at will be (for engine cooling reasons at least) way higher than Vs. So if you set it to say 90kt, it won’t stall but the ceiling reached will be whatever corresponds to a 90kt IAS cruise, which is not a lot…

Administrator
Shoreham EGKA, United Kingdom

If you engage pitch hold mode with a nose-up pitch angle that is greater than the critical angle of attack, the aircraft must eventually stall.

The angle of attack is of course the difference between the climb angle and the pitch angle, so while in a (sufficiently steep) climb the wing is not stalled.

But eventually the aircraft will stop climbing and reach level flight. At that point the wing will be stalled.

Of course in practise the aircraft might stall before reaching level flight and also it could stall even if the nose-up pitch angle is not greater than the critical angle of attack, but this example shows that it is possible to enter a stall from a climb with the A/P in pitch hold mode.

ESKC (Uppsala/Sundbro), Sweden

EuroFlyer wrote:

What do you mean by stall critical angle ?

(simplifying a bit) wings tend to always stall at the same angle of attack, independent of airspeed. My thinking was that if we are flying level than that angle of attack is simply the angle of the wings to horizontal. If we start off at a small angle of attack, and all that happens is that the climb rate decreases, sooner or later we will be flying level – and at that point, we would be in equilibrium.

However, I suspect @RobertL18C has hit the nail on the head: as the climb slows the angle of attack increases Increases. Increased angle of attack means more drag, so our climb slows even more, at some point (if the engine isn’t powerful enough) we get on the back side of the drag curve – and then we start to descend, which only increases the drag and makes it worse.

So maybe it’s fair to say that there might be a low pitch angle at which the level flight equilibrium will be reached before we start going up the back side of the drag curve – but we must absolutely not rely on it.

EGEO

We typically use PIT (pitch) mode when climbing above FL280 for pax comfort because the aircraft tends to pitch a lot in FLC (Mach hold).
While holding 2.5 degrees pitch up on the AI, our AoA (Angle of Attack) indicates an increase from around 22% to 30% of Max as we climb to the mid 400 levels, and the IAS bleeds off. These figures vary with weight and temp conditions.

ESSB, Stockholm Bromma

The only scenario with Pitch Hold climb and full power that will not eventually lead to a stall is when te set pitch angle is low enough that the airplane will eventually reach an equlibrium in level flight with an angle of attack that is smaller than stall. As the angle of attack, as pointed out above, is the angle between the chord line and the free airstreem, it means that if the set Pitch angle is smaller than stall angle in realtion to horisontal, and the engine has enough power to maintain the resulting airspeed, it will just maintain that and will not stall. However, the resulting airspeed could be below the minimum airspeed for which the autopilot will work, meaning it may not have authority to keep the wings level.

While the VS mode should in theory always reach stall eventually (when set to a climb!), pitch hold should not do that if set to reasonably benign climb attitudes.
This is all from theory – while I have tested VS mode and repeatedly found it dangerous as hell if not watched carefully, I have not yet tested pitch hold.

huv
EKRK, Denmark

The DA62 will fly a go-around on the autopilot because the installation in the GFC700 will not stall in pitch-hold mode because USP will kick in (under speed protection).

Obviously you need to apply power (no auto-throttles). It was the addition of USP that got this through certification, otherwise you might end up in a stall very close to the ground if you forgot to apply power…

EGTR

nokicky wrote:

The DA62 will fly a go-around on the autopilot

You mean fully coupled ie you never disconect the A/P during the go around?

EGTK Oxford
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