Speed trend indicator

[MHN_pilot]

Hi!

I wanted to ask you about this aid,

-How does it work in a modern a/c such as a320, is it air data or inertial related??
-I had a little experience in citation III sim but never knew how to take advantadge of it. Is it useful for you?:ugh:


Thanks for your input!

Happy landings.

[Checkboard]

Yes, it's useful. Generally the EFIS system calculates the speed the aircraft will be at in 10 seconds, at the current rate of acceleration.

It is good for ensuring the power selected is correct to hold a certain speed, when managing the engine power manually.

[Badente]

Not an A320 driver but in a B737 it is close to useless when operating in bumpy/gusty conditions. Jumping all over the place from upper end to the lower end of the speedtape while the speed itself varies less than 5 kt in each direction. I found flying pitch & power leads to an overall more stabilized approach. Maybe a matter of habit.

Regards, Badente

[Microburst2002]

Interesting question

Recently I have been wondering if the trend vector is based on air data (IAS) or inertial data (G/S) or what.

The trend vector is very useful for handflying. You have to scan the N1 or EPR gauge less frequently because you can "feel" when the thrust target is getting close to the required one, or too short, or too far.

[safetypee]

Micro, IIRC (based on speed trend systems which I used and evaluated), the computation involved a mix of speed and inertial inputs.
The air data required ‘just the right amount’ of damping – to cut out the turbulence, but not to be sluggish.
The inertial input (acceleration?) required some thought as to which axis to use, normally the aircraft flight path, and again with appropriate filtering / damping.
Memory fails me for details, but I recall that a careful evaluation was required during unusual situations, particularly stalling and in windshear. The problem was that in some situations the air data speed could be moving in the opposite sense to the inertial data, e.g. aircraft speed decelerating, but accelerating along the flight path. Such a situation was not acceptable to the regulatory authorities.

Speed trend originated appears to have originated from avionics designers – "because it could be done - it’s a good idea", but IMHO it is a necessary supplement to a speed tape display which sacrifices many of the positive display features of a dial.

[Microburst2002]

thanks

so, is there any inertial "contamination" in it?
or it is pure air data?

If the air data is wrong, the trend is wrong, too, right?

[Capt Turbo]

It seems as if the B737 inertia damping is less than on the Airbus, hence the remark about it being erratic and useless.
Especially on a heavy Airbus (or 747?) it is the last line of defense against a wrongly calculated reduced takeoff thrust. 4-engine aircraft is normally taking off with a reduced thrust which only leaves them about 10% runway left after becoming airborne.
With a lift-off speed of say 160 KIAS or 160 knots ground speed in calm wind I am sure that someone here can calculate the acceleration required to become airborne in 3000m/10.000´. Anyone???

So, as a rule of thumb on these birds; if you do not have a steady acceleration of at least 20KTS/10 sec shown on the speed trend vector - GO TOGA or abort the take off or you will - at best - be scraping the approach lights in the far end as have been seen a few times.

[safetypee]

Micro, right, trend wrong!
Certification requirements prohibit the display of hazardously misleading information; thus if the ADC fails – no airspeed, the airspeed display should be removed (some older systems flag the scale), which means that the speed trend should similarly be removed as there is no reference scale.

Re inertial ‘contamination’. System designs vary; where IRS is available then some form of acceleration might be used. If FPA is computed then this could the reference axis for the acceleration. IIRC, other, lesser systems used a stand-alone accelerometer, not seen much nowadays.

[Microburst2002]

thanks again, safetypee


Capt turbo

in those birds of yours is where all that stuff about take off performance becomes really meaningful...


cheers

[shortfuel]

Yes, it's useful. Generally the EFIS system calculates the speed the aircraft will be at in 10 seconds

You meant the Flight Augmentation Computer...

[safetypee]

Depends on the system design / aircraft type. The aircraft which I referred to did not have a FAC, instead a ‘system’ which provided data collection and formatting (some computation) for display on a tube – Electronic Flight Instrument System.

Beware single type, single mind design theory.

[shortfuel]

Sorry...but I was quoting Checkboard, not you.