Transponders and ACARS - Priority Systems

[BM82557]

I'm curious as to why critical aircraft systems, such as transponders and ACARS, are designed so that they can be switched off by the flight crew or others. I would think that something of that importance would be on permanently whenever power is available and off only when the entire aircraft is powered down. Any ideas?

 

[kma371]

My guess is what happens when its malfunctioning? There needs to be a way to turn it off so as not to disrupt radar,etc.

 

[DaveNF2G]

Exactly. If something is blowing fuses, they need to be able to turn it off. Ditto if it gets stuck in transmit mode.

 

[profiledescent]

Fire in an aircraft is not something you want. Everything, and I mean EVERYTHING aboard an aircraft has to have the option of being shut down or de-energized.

 

[nr2d]

At 1 time it was SOP for pilots to switch off their transponders while taxiing and it would be switch on just prior to takeoff and it would be switched off after exiting the active runway. Now pilots at certain high volume airport are required or requested to keep their transponders on. This is because of the new ASDE system installed at the airport which give the controllers a display of the taxiing aircraft on the tower radar display in poor weather.

As for the ACARS I don't know if they can switch it off. I've never worked on the aircraft equipment or ground equipment for the system. If you are referring to the missing aircraft I think their ACARS system was working after the transponder was turned if I read the news reports correctly.

As someone above said the pilots have access to all the circuit breakers for the systems in the aircraft. The ACARS system would be on it's own circuit breaker.

 

[profiledescent]

ACARS can be controlled by the crew.

 

[HopperD]

I read somewhere that pilots turn them off when at airports because it clutters the screens for ATCs, which makes sense.

 

[profiledescent]

I read somewhere that pilots turn them off when at airports because it clutters the screens for ATCs, which makes sense.

That is exactly why. As technology has improved, and with the advent of Mulitlateralization sensors on the ground, at many airports they are requested to leave it on for ground surveillance as well. At those airports, the surveillance RADAR and the ground system (ASDE) have been integrated in a way that eliminates the "ground clutter" or "ring around" issue at the airport RADAR site.

 

[ai8o]

1) You don't want electronic devices on when you are powering up an aircraft. Switching from an APU, or starting from full off will generate electrical transients.
Transients will scramble already programmed parameters. Repeated transients will kill avionics.

2) An APU cannot carry the entire load of all the motors and avionics on an aiplane.

3) You want as little load on the onboard generators as possible when you are first rotating the power generating equipment. A full load on an onboard generator is analogous to trying to start a diesel locomotive with the dynamic brakes fully engaged.