Autopilot IAS conventions

[Bert Pieke 7,584]

I wonder if a RW pilot could help me figure this out..

In an aircraft with IAS mode on the autopilot (airspeed hold by controlling pitch, ie no autothrottle),

what are the conventions for the UP/DN rocker?

Lets start with VS mode:   UP is Nose Up, DN is Nose down.  No dispute.

In IAS mode, I would expect the same convention to hold: UP is Nose Up, DN is Nose down.

However, some addon aircraft instead have: UP is Speed up, DN is Speed down, which is the opposite... confusing to the pilot I would think..

Anyone care to comment?

[Chock 17,769]

As you can imagine, there is no official convention for that kind of thing, it is up to the aircraft and avionics manufacturer. However, you might be interested to know that one or two major airlines have in the past been known to go to the expense of having some control switches reversed in operation in order to have them match the operation of other aircraft in their fleet, I seem to recall United did that on their 727s for one or two controls, although it might have been one of the other big US carriers.

These days, that sort of thing is being taken into account more with haptics and ergonomics intended to be intuitive. Indeed even such things as cockpit designers being aware that a new generation of pilots will be familiar with stuff like X-Box controllers is having a bearing on how controls are laid out (yes really!). This isn't a new concept actually, the CDUs which first appeared in the Seventies were deliberately designed to look like the pocket calculators which were also showing up around that time in order to convey the notion to pilots that its use involved important calculations. And like the typewriter keyboard, which was designed to slow people down a little to avoid them making typos, the CDU was also designed to be slightly convoluted in how you use it, in order that things would be keyed in a bit more slowly to eliminate the possibility of rushing stuff and instead putting inputs into the correct section. So the next time you think putting stuff into a CDU is a pain in the arse, you can thank NASA for that one, because it was their idea!

[JYW 2,978]

Not a licenced pilot, but in the rw aircraft that I've flown in that have IAS mode (linked to an UP/DN rocker, rather than a dedicated speed knob), the UP would always decrease IAS, (as UP would still refer to "nose up" - a reduction in airspeed) and DN would always increase IAS, similarly because lowering the nose would increase airspeed.

This goes for the GFC700 (G1000) and the Meggitt Magic 2100 - both of which have IAS mode and UP/DN rocker switches (separate buttons in the case of the GFC700).

[Bert Pieke 7,584]

Thanks, that is my interpretation as well.

[JYW 2,978]

20 hours ago, Bert Pieke said:

Thanks, that is my interpretation as well.

I do think a number of addon developers have gotten confused however, and assumed that Up means "Speed up" and Dn means "Speed down" !    Certainly that seems to be the case in at least some Carenado addons.

[WarpD 824]

While the correct behavior is based on what the avionics manufacturer has chosen... I can understand the confusion.

However, I will point out that with IAS hold... throttle controls "up/down" far more than the airspeed.  So... perhaps to some avionics companies that is why up increases the speed value and down decreases it as their thought is that throttle controls pitch.

All a matter of perspective.

For a Pro Line 21 system... there is an independent knob used to control the value for FLC (the IAS hold).  Helps eliminate confusion if you ask me.

[Bert Pieke 7,584]

17 minutes ago, JYW said:

I do think a number of addon developers have gotten confused however, and assumed that Up means "Speed up" and Dn means "Speed down" !    Certainly that seems to be the case in at least some Carenado addons.

Indeed.. that is what prompted my question..

I have changed it in my PC12 V2 care package.

It feels more intuitive to me that UP is always Nose Up, regardless of autopilot mode - but that may just be me.

Maybe the Swiss (Pilatus) feel differently.. 

[Bert Pieke 7,584]

4 hours ago, WarpD said:

 

However, I will point out that with IAS hold... throttle controls "up/down" far more than the airspeed.  So... perhaps to some avionics companies that is why up increases the speed value and down decreases it as their thought is that throttle controls pitch.

 

Ed, I am sure you are right.  The correct way to fly the airplane is to engage IAS as speed hold, and use the throttle for vertical control.

Still leaves the question of which direction the airplane moves when adjusting the rocker switch up in the air, so to speak..

The RW KMC321 manual is not really specific..

"Indicated airspeed hold:Adjusts the indicated airspeed at a rate of 0.75 knots per second until released.

Vertical Speed hold: Adjusts vertical speed at a rate of 100 fpm per second until released."

[Gregg_Seipp 568]

On 3/9/2017 at 0:03 PM, Bert Pieke said:

In IAS mode, I would expect the same convention to hold: UP is Nose Up, DN is Nose down.

Somebody can correct me here, but IAS mode in a non-autothrottle aircraft, as far as I've seen doesn't have anything to do with nose...only speed.  I think the Flight1 Mustang is a good implementation.  If you're cruising along at 160 KIAS, set a higher altitude and punch IAS, it won't climb.  if you roll the IAS speed up to 170 without adjusting throttle, it'll actually nose down, descending, attempting to capture 170 KIAS.  If you, instead, increase throttle it will start a climb to capture 170.  It will stay engaged until it captures the ARMed altitutude and then automatically switch IAS mode to ALT mode.  Most folks don't use IAS mode on descent but it would be useful if ATC gave you a speed to maintain and a descent...set a lower altitude, set IAS, set the speed, throttle back.

[JYW 2,978]

1 hour ago, Gregg_Seipp said:

Somebody can correct me here, but IAS mode in a non-autothrottle aircraft, as far as I've seen doesn't have anything to do with nose..

This is essentially true, but on autopilots without a dedicated "speed" rotary knob (as Ed is referring to above), the selected airspeed is often controlled via the UP and DOWN buttons/rocker.   Bert was asking whether UP decreases the airspeed value or inceases it, and the same for the DOWN button.

Assuming throttle/power settings are unchanged, increasing the target IAS value will always result in the nose of the aircraft lowering, and decreasing target IAS values will always result in the nose raising.   Of course if power settings are changed, any dynamic with the nose could occur, as the aircraft pitches to maintain IAS according to the power setting.

[Gregg_Seipp 568]

2 minutes ago, JYW said:

This is essentially true, but on autopilots without a dedicated "speed" rotary knob, the selected airspeed in controlled via the UP and DOWN buttons/rocker.

Yeah, that's how it is on the mustang...no dedicated speed control.  If you're in VS mode the rotary thingy works for that...if you're in IAS mode it's for that.  I'll have to check to see which way it goes with the mouse wheel in each mode...not sure if would be the same as the PC12 tho.

[FormerSF3 15]

Bert.  On the MD80 it works like this:  When IAS is selected, rotating the wheel up increases the speed, rotating down lowers it.  Its the same for the VS mode.  The nose is going to follow the direction of the wheel.

On the Saab 340 I flew with the Collins autopilot, I could select IAS mode, if I moved the UP/DOWN rocker it would revert back to VS mode.  Then the nose would follow the direction of the rocker.

[Bert Pieke 7,584]

24 minutes ago, FormerSF3 said:

Bert.  On the MD80 it works like this:  When IAS is selected, rotating the wheel up increases the speed, rotating down lowers it.  Its the same for the VS mode.  The nose is going to follow the direction of the wheel.

 

Thanks! That would support my theory,  "rotating the wheel up" = DN on the control and lowers the nose, increasing the speed. 

[FormerSF3 15]

Yes.  After reading my own post, I don't think I explained it as well as you did.

[vololiberista 205]

Where you have dedicated manometric locks. For example IAS you should also have a datum adjuster. This will work also for altitude hold. Where it is used to increase/decrease the rate of climb/descent. In the case of IAS the AP will try to hold the acquired IAS and then with the datum adjuster the crew can fine tune it. Early APs were prone to porpoising so the technique was to set an airspeed 20 to 30 kts below the target and then adjust up to say 250kts.