NAV engaged on ground - A320

[CaptainMongo]

I had an aircraft where NAV indicated engaged while at the gate. We cycled the FD's. NAV then indicated armed but then immediately indicated engaged. We cycled the NAV data base dumping the FMGC and reinserted the flight plan. After reinserting the flight plan, NAV once again indicated engaged. At that point I was ready to dump the IR's and re align. It just so happened at that time the ground staff wanted external power disconnected. We did so and the power interruption/switch over fixed the NAV engaged problem and the FMA's indicated properly.

What was the reason for this anomaly?

[Amadis of Gaul]

Some ones and zeros crossed incorrectly.

[tpng conehead]

Dirty external power will cause problems with digital systems.

[Capt Quentin McHale]

Conehead,


In all my years I have never heard the expression "dirty external power" before. Could you please explain. Thanks in advance.

[Lucky8888]

An abnormality in the power quality that is being delivered to a system is often referred to as "dirty power." Abnormalities can include low power factor, frequency variations, voltage variations, and surges/spikes.

[Bula]

Can happen if you inadvertently delete the departure threshold in the MCDU using a Direct-To or the like. FMS goes into climb mode and can engage NAV on the ground.

I wonder if pushing the EXT PB wokeup the ground logic.

[Uplinker]

An abnormality in the power quality that is being delivered to a system is often referred to as "dirty power." Abnormalities can include low power factor, frequency variations, voltage variations, and surges/spikes.

A more likely feature of 'dirty power' would be electronic noise or interference frequencies superimposed on the line voltage. These could be caused by faults in the power source generating circuits. The thing is; the line voltage and frequency might be perfectly correct, but if it has noise on it, (which an avionics engineer would be able to see on an oscilloscope), a digital system can get confused because noise can appear to be a random series of Ones and Zeros.

Having said that, I have never known this problem, but I have seen the power transfer from external power to APU, (or vice versa), sometimes cause faults in some systems.

[Amadis of Gaul]

Conehead,


In all my years I have never heard the expression "dirty external power" before. Could you please explain. Thanks in advance.
I have. Also, terms like "screwy", "iffy", "garbage", and "bs" are used occasionally.

[hph304]

Had this problem once or twice, but after engine start the FMA reverted to NAV blue.