737NG Airspeed Trend Vector

[QAVION]

A question for the NG technical experts...

Would the Airspeed Trend Vector on the PFD MASI use acceleration data from the IRU's or air data from the Pitot-Static system?

All the Boeing MM seems to say is that is comes from the DEU. Is it a combination of both, perhaps? (like the IVSI's on some aircraft)

Has anyone pumped up the pitot tubes during ground maintenance whilst observing the MASI? (i.e. no IRU input)

Thanks.
Regards.
Q.

[avioniker]

Good question. We just ran a plane up doing an overspeed check. I'll have to go back and look again. I don't remember the trend vector being there. It would make sense that no trend vector could appear without inertial data.

Here's what the SDS section 34-21-00-010 has to say:

"DEU
The DEUs use this IR data from the ADIRUs to show information on the display units:
Present position, Track angle, Magnetic heading, Wind direction, True heading, Ground speed, Drift angle, Flight path angle,
Flight path acceleration,
Pitch angle, Roll angle, Body pitch rate, Body roll rate, Body yaw rate, Body longitudinal acceleration, Body lateral acceleration, Body normal acceleration, Track angle rate, Pitch attitude rate, Roll attitude rate, Inertial altitude, Along track horizontal acceleration, Cross track horizontal acceleration, Vertical acceleration, Inertial vertical speed, N-S velocity, E-W velocity."

and here's SDS 31-62-00-301:

"Speed Trend Vector
The speed trend vector shows the predicted airspeed in 10 seconds based on current acceleration or deceleration. This vector shows when the speed trend is more than 4.5 knots."
:D

Sounds like a combination of elements to me...

[QAVION]

Thanks Avioniker.

Apart from having actually done an overspeed test, I seem to have all the data you have. The DEU's, of course, have all that IRU data for a thousand and one purposes, but our books don't always tell us what each bit of data is used for.

I would have leaned the other way towards purely airspeed, otherwise the trend vector would no longer represent airspeed, but an increase in aircraft inertial acceleration. The aerobaticists amongst us can probably tell us the difference between the two and how it affects aircraft behaviour.

If the trend vector was purely inertial, what indications would you get on your MASI if an aircraft was accelerating along the runway and was suddenly hit from behind by a large gust of wind? Would the indicated airspeed rate of increase reduce or stop and the trend vector remain at the same magnitude? (or even increase due to the force of the wind).

Thanks.
Regards.
Q.

[avioniker]

If all goes well we're doing a post rudder mod test flight today PM or Monday AM.
Will advise.