A320 EXP DES - V/S 0 - PFD Speed Trend Vector Peculairity

[Bula]

An unusual behaviour which I have never been able to put my finger on.

If one is in EXP DES and pushes V/S 0, even with a decelerating airspeed, the trend vector heads north until its off the airspeed scale.

Does anybody know why?

[Ollie Onion]

Because the plane is thinking 'what the are you doing?'

[Bula]

I thought about that one, atleast the feeling is mutual

[clark y]

Bula,
You've probably give the computer a Vestibular or somatogravic illusion while trying to do a 1.2 or is that a 1.3g level off.
Another fun one I've noticed (Airbus) is how the wind vector changes when taking off or turning in the circuit areas. I've always put it down to the relatively high rates of change whether it be acceleration, angle etc.

[PilotJames]

Bula, surely when you press EXP DES you get a positive trend vector as the aircraft starts to accelerate. When would you have a decelerating airspeed vector in EXP DES?

[FlightDetent]

Not in EXP DES. I read it he'd use the EXP DES to send the A/C in a dive, then push V/S=0 and as the pitch comes up the speed starts to drop...

[Lantirn]

If one is in EXP DES and pushes V/S 0, even with a decelerating airspeed, the trend vector heads north until its off the airspeed scale.

I have noticed the same thing. It happens during ALT*

I have no idea but it should be due to the longitudinal acceleration component during ALT*

[Bula]

You are correct. EXP DES then VS 0. Pitch up, aircraft pitches, aircraft decelerates.

Thanks for the answers people.

My thinking is given the trend vector is IRS based, I'm guessing the constant Pitch G is being mistaken for a constant acceleration G. Though this is assumption, it seems we are on the same page.

It always amuses the person next to me.

[Lantirn]

It's not that simple as speed trend is a mix of IRS data and airspeed data. I have no reference but I suppose the initial calculations should be from IRS.