Alternate Static Source Backwards?
I have a 78 Turbo Arrow with what looks to be a factory alternate static source installed. I think it might be plumbed backward.
I was trying to get to the bottom of some optimistic cruise speed indications (see "Turbo-normalize Arrow?" thread for some back & forth. One poster suggested that I consider that there may be a leak in the static system.
In the meantime, flying some wind boxes indicated that IAS is about 5-6 knots high in 65% cruise.
The placard for the alternate static source states that the lever should be pulled aft to activate. I had it forward since I bought the aircraft in January. I closed all the vents and pulled it back and didn't get much of a response from the instruments though the VSI steadied out on a descent. I then popped the side window and opened/closed vents expecting to see the Altimeter, VSI, & ASI jump. Nada. I closed the window & vents and then pushed the lever forward. I got what one would expect from using the alternate static source:
Altitude - up a couple hundred feet, VSI stabilized in a climb then zero, and an increase in indicated airspeed.
Am I missing something?
Thanks,
I was trying to get to the bottom of some optimistic cruise speed indications (see "Turbo-normalize Arrow?" thread for some back & forth. One poster suggested that I consider that there may be a leak in the static system.
In the meantime, flying some wind boxes indicated that IAS is about 5-6 knots high in 65% cruise.
The placard for the alternate static source states that the lever should be pulled aft to activate. I had it forward since I bought the aircraft in January. I closed all the vents and pulled it back and didn't get much of a response from the instruments though the VSI steadied out on a descent. I then popped the side window and opened/closed vents expecting to see the Altimeter, VSI, & ASI jump. Nada. I closed the window & vents and then pushed the lever forward. I got what one would expect from using the alternate static source:
Altitude - up a couple hundred feet, VSI stabilized in a climb then zero, and an increase in indicated airspeed.
Am I missing something?
Thanks,
Comments
I don't believe the outside source is blocked, IAS, Altitude, & VVI all work "normally" in either condition - that is to say that the IAS does not behave like an altimeter, the altimeter indicates higher when I go higher, and the VVI assumes the right trend. The instruments just behave as if they're on the alternate static source when the switch says not.
A puzzler.
It is rather unlikely the switch/valve is wrong as it mounts with 2 screws and regardless what you do to it the on/off direction would not be impacted.
Regards,
Joe
It is possible that the valve was installed incorrectly (or has moved) on the bracket. Only one of three possible positions will result in the valve opening when the "lever" is pulled "aft". That bracket is also mounted on an angle making interpreting what "aft" is confusing too.
An open Alt Static Source generally indicates higher altitudes and higher IAS and a momentary climb on the VSI when first opened (and the opposite when closed).
Figure out which position is open and which is closed and make a yourself a more understandable placard.
PilotKris
I do like the slamming the door test - ridiculously simple and no flight time required.
> Thanks Kris. I'm thinking you were right on the
> money regarding a static source problem as a
> culprit for the high ASI numbers a while back.
> I'm going to sort through this and then fly
> another wind box. If you're interested I'll let
> you know where it actually trues out if I can get
> consistent numbers.
>
> I do like the slamming the door test -
> ridiculously simple and no flight time required.
Glad I could help.
I would be interested to see how it all really works out. I'd expect about 134kts IAS at 75% (regardless of altitude).
Once in a while as a diagnostic tool is OK but "slamming" aircraft doors is a pet peeve of mine. The vast majority of airplane doors I've seen have been tweaked by people slamming them as if they were a car.
My father taught me almost 35 years ago the proper way to close a (late model) Piper door.
From the co-pilot seat.
1. Lift the lever with your right hand
2. Gently pull, do not "slam" the door shut with your left hand
3. Push down on the lever with your right hand latching the door
4. Latch the upper lever.
We've been closing our Turbo Arrow door that way since new in '77 and the door is as straight a true as the day it came out of Vero Beach.
PilotKris
> And you slam earlier Arrows and Cherokees as there is no locking lever mechanism to latch them closed.
Er, I instruct my unfamiliar passengers to "pull firmly" rather than to "slam", but your point re: different latching mechanisms is well-taken... : )
(Anyone ever notice that the door latch handle for older Cherokees appears to be identical to the one used on VW Beetles from the same era?)
The previous owner emphasized door closing technique when I picked up the plane. I had my CFII along for the long trip from Louisiana to Maryland. When we manned up to leave, he promptly reverted to the 'slam' technique used on the '68 rental Arrow we've flown a lot. There was a short silence followed by, "I won't do that again." The door survived & it was a funny moment. I do like the door on the '78.
> I do like the door on the '78.
Later doors are much nicer.
jruhnke Wrote:
> I instruct my unfamiliar passengers to "pull firmly" rather
> than to "slam"
And when you have to do it a second time "pull harder."
I suppose even a broken clock is right twice a day ;-)
Thanks,