Low cost air speed indicator.
Not compensated for temperature or altitude.
Useful in the back seat of a tandem aircraft without
 a rear instrument panel.
It is a simple water manometer 20 inches high.
Paul Lamar

Since one is most interested in accurate airspeed when on final, would
the errors tend to cancel out changes in aircraft performance due
to  temperature or altitude?

If the air was denser, the manometer/altimeter would read a higher
airspeed (correct??), so the pilot would unknowingly allow the aircraft
to fly slower to "normal" indicated airspeed.  But when the air is
denser, the aircraft will also fly slower/ground effect will increase/
etc, and the pilot should be able to make a safe "normal" landing.

Or am I thinking backwards here?

Robin B

I am not sure I understand what you are saying "changes in A/C performance".
Do you mean stall speed? I have heard of a small airline pilot in
Mexico that would stall the old twin piston engine airliner (at
altitude) just  before landing on a short dirt strip to make sure he
landed it as slow as possible.  Paul Lamar

"Stall speed"?  Basically ... Yes.  My old CFI kept hounding me about
landing too fast.  Even if I greased  a landing (but still had too much
speed) he would pull back on the yoke and force the aircraft to lift
off the ground again to demonstrate that I had come in too fast.  (I
think it mostly had something to do with the fact that the old C150
he used to rent to me for $11/hour had a nose-wheel that would
almost shake off at speeds above 30kts :))

So if you make a landing using one of those manometer airspeed
indicators on the approach, and it happens to be a cold day, the
dense air will push the water in the manometer higher, indicating a higher
airspeed.  The pilot will then force the aircraft too lose more airspeed
and land the aircraft at a lower true speed (But the aircraft can handle
it, because it will not stall before touchdown because the air is colder
and denser on this day.

Conversely ... On a hot day the aircraft will not fly as well, the
air is less dense.  On approach the manometer airspeed indicator
will not read as high due to the light air; the indicated airspeed
will be lower than actual. The pilot will adjust power/attitude to
keep the indicated airspeed at what he thinks is correct but is
actually above desired airspeed, but in the light/thin air more
airspeeed is required for a safe landing, so everything works out fine.

Make sense??

Robin B


A water manometer is open on both ends, thus the air density has no effect on
the reading. The end open to cabin air pressure as static can affect it some.
If the cabin is pressurized the static end would have to be plumbed to a still
atmospheric air place in the plane. Also good as a backup. Thinner air may not
move the water column as far as denser air, but the effect of variance would
be more pronounced at higher velocities. Should be minor at landing speeds.

Dale Davies