Two questions

[Eric]

I know cranking the engine with the avionics on can damage them. Can brining the mixture to idle cut off before turning off the avionics send them a surge of power?

Does indicated maneuvering speed change with altitude? I took a check out at a high altitude airport and was told that Va was higher due to the altitude.

Thanks again!

 

[banned username 2]

I know cranking the engine with the avionics on can damage them. Can brining the mixture to idle cut off before turning off the avionics send them a surge of power?

It is always a good idea to shut off your avionics before changing the bus voltage (i.e. going from 28v alternator to a 24v battery)

Does indicated maneuvering speed change with altitude? I took a check out at a high altitude airport and was told that Va was higher due to the altitude.

No, Indicated Va is based on dynamic forces which are created due to the dynamic pressure of a particular speed through a fixed amount of air. As your altitude increases you have to cover a greater distance to "hit" as many air molecules. I think what the instructor meant (or should have said) is Va (True Airspeed) increases with an increase in altitude, but Va (Indicated Airspeed) stays the same. The reason Va (TAS) increases with an increase in altitude is the same reason your cruising TAS increases with an increase in altitude.

Hope this helps... Man, it has been a while since I taught this stuff!

 

[avbug]

Eric,

In the situation you describe, potential avionics damage can occur due to voltage spikes and transients. Your avionics are very sensitive, and susceptable to damage. Voltage transients can occur faster than the regulator has power to control them. When starting or shutting down, turn off your avionic individually, or by the avionics master.

Some equipment is more susceptable than other equipment. Don't take chances. Shut it off. Even if you're using external power, you can still experience spikes and transients in your power bus(es), and damage can occur long before a breaker can pop, or the regulator can catch it. (Remember that circuit breakers are there to protect the wiring, not the equipment in the circuit).

Maneuvering speed changes with an increase in weight, increasing with weight...not with altitude.

 

[midlifeflyer]

Does indicated maneuvering speed change with altitude? I took a check out at a high altitude airport and was told that Va was higher due to the altitude.

Actually it does. Sort of.

Load limited =indicated= airspeeds don't change with altitude Stall speed, best glide, 1.3Vso, maneuvering speed, all keep the same =indicated= airspeed. (although also load-limited Vx and VY do change, but for different reasons).

The kicker is that those same =indicated= airspeeds translate to higher =true= airspeeds. So a 100 kt indicated Va at sea level density altitude is also 100 kts True, while at 12,000 DALT its about 120 True.

I wonder whether the person who made this comment had a small understanding of this and it lead to the comment you mentioned. While true, it's pretty irrelevant. ("We've hit severe turbulence! Quick! Look at the OAT! Reset the altimeter to 29.92!Pull out my E6B! I need to figure out what speed to fly!")

 

[Eric]

Thanks guys. Confirmed what I knew about Va and altitude. I knew the TAS went up but figured it was irrelevant due to Falcon Capt.'s dynamic pressure explanation. He was trying to explain what the differences were at a high altitude airport.

Makes sense about the bus voltage changes. I will incorporate this into my teaching. The 1969 C-150 checklist we are working off of says:

1)Mixture - idle cut off
2)All electrical switches off

Now I know why the guy freaked out when the first thing I did was pull the mixture