Autopilot Screwup?

[forfun]

Hey
k, im flyin along right, and decide i cant be botherd controlling the plane, so i turn on the alt hold, ias hold, and heading hold. This works great when cruising, but in a climb the autopilot stalls the aircrafty. Its SOO annoying

Anyone know what's going on here, it happens at high altitudes and it cant be the mixture cos i have automixture on always

cheers
forfun

[Skligmund]

Yeah, one of four things:

1. You have autothrottle on, and the airspeed is set to zero.

2. You do not have a throttle position adequate for your climb rate.

3. You have you vertical speed bug set too high of a climb rate.

4. You are trying to climb to an altitude which you cannot obtain with that aircraft.

or a combination of the 4.

[Nav]

forfun, so difficult guessing if you're fairly new to this or a 1,000-hour veteran  :)

Assuming the former, remember 'power first, then pitch'.  If you want to climb, the FIRST thing is to add power; only pitch the nose up when you've done that.  With low- powered aeroplanes like the default Cessna, you'll need full power to climb in any conditions.

Second, the rate of climb is important - and the achievable rate varies with height, as the air gets thinner.  The Cessna will climb out after takeoff at 800 feet/minute, but above about 5,000, 500/minute is about the limit.

At the other end of the scale, a jetliner will climb out at just about any rate you like at first - but above say 20,000, 1,500/minute is about the limit.

Mainly, watch your airspeed - keep it at sensible levels.  If it looks like dropping into the danger zone, reduce the rate of climb till it gets back to a safe figure.  In fact, holding the right speed is your main concern when climbing - if you look on the 'kneeboard', or in the 'Flight Notes' for the aeroplane you are flying in the Learning Centre, it will tell you the recommended climb speeds.

Hope all that helps  :)

[forfun]

Hey

I 4got to say this only happens in the jetliners. Nav i think your right, i just thought that an autopilot would pick these things up rather than increasing pitch and stalling the aeroplane. Although in the light aeroplanes i don't even use autopilot so that's probly it.

I also found in the 737 last night i was climbing to FL350 at a vertical speed of 600fpm it still stalled the aircraft. I assumed this was to do with the mixture cos i was losing power fast

cheers
forfun

[MattNW]

Hey

I 4got to say this only happens in the jetliners. Nav i think your right, i just thought that an autopilot would pick these things up rather than increasing pitch and stalling the aeroplane. Although in the light aeroplanes i don't even use autopilot so that's probly it.

I also found in the 737 last night i was climbing to FL350 at a vertical speed of 600fpm it still stalled the aircraft. I assumed this was to do with the mixture cos i was losing power fast

cheers
forfun

Try using a lower vertical speed on your AP. The jetliners really depend on weight as to their climb rate. If you are a little heavy then you need to climb slower than with a light plane. There's also the factor of altitude. If you are at FL 20 climbing to FL 30 then you won't want to climb at maximum rate like you may for a climb from 3,000 ft to 4,000 ft.

[Nav]

forfun, mixture settings only apply to piston engines, not jets.

Sounds like speed (or lack of it) is part of the problem.  How are you setting it?  In your first post you mention 'IAS', Indicated Air Speed - this is inadequate above about 20,000 feet because of the thinner air; you should switch to setting the speed in Mach speed (i.e. true airspeed); say about Mach 0.65 in the climb.

Another gauge to keep an eye on is 'N1'; engine power as a percentage of full power.  Keep an eye on it in the climb, and fine-tune the rate of climb to keep it around say 80%.  

It's all a matter of 'balance' - if you set too high a speed, the autopilot/autothrottle will use all the available power to maintain the speed, and have nothing left to climb with; if you set too steep a rate of climb, the auto-controls will point the nose higher and higher trying to maintain it, and you will lose flying speed.  Either way the result will likely be a stall.

Strongly recommend that you look up the Flight Notes in the Learning Centre for any aeroplane you plan to fly; they're not perfect, but they'll give you a lot of the key information.

Hesitate to suggest this, because you'll need to persevere, you'll probably have a few 'false starts'.  But try going to Flying Lessons, Airline Transport Pilot, Lesson 1-Jet Checkout.  In that, Rod Machado will 'talk you through' a full takeoff and landing in a 737.  It requires fairly accurate flying - he may stop the lesson a few times if you get too far off the speeds, heights, and rates of climb he asks you for; but if you can stick at it and get through it, you'll know an awful lot more about the whole business, including which instruments to keep an eye on.

[Nexus]

Since a jet engine is most efficient around 95%, there's no reason to keep it at 80%. A turbofan engine is designed to run at high rpm's. Keeping it at 80% is like driving a car in the 4th gear at 30mph....

And mach number (MN) is usually used above 26.000ft because MN becomes the aircrafts limiting speed reference. Up to ~FL260, you will climb with a  constant IAS against an increasing MN.
when passing thru the crossover altitude (also know as changeover) the aircraft is flown at a constant MN with a  decreasing IAS for an increase in altitude.
Airliners have two sets of climb speeds; one in IAS and one as mach. When the mach climb speed equals the IAS climb speed you make the crossover.