Airspeed Indicator Accuracy Tolerance

In my Cessna 421C, the pilot-side airspeed indicator reads 6 nts below
the co-pilot-side airspeed indicator. When I pointed this out to a
mechanic, he said that it was "within toleraance." He said it was
possible that one was 3 nts high and the other 3 nts low and so both
could be within tolerance. He added that there is no real internal
adjustment, so there is not much he could do about it anyway.

Seems to me that when you are talking about a possible 6nt difference
and given the need for accurately maintain blue-line speed in case of a
single-engine failure, it is important to know that one's airpseed
indicator is accurate. Isn't there a way to judge true accuracy of
speed indicators (just as there is for altimeters)? Should I be
satisfied with my mechanic's answer and move on?

-Sami
N5554G, Cessna 421C

If there is no means of adjustment, the question answers itself. Keep in
mind that the blue line is not sacrosanct...actual Vyse will vary with
weight and density altitude. The vertical speed indicator is a much more
accurate means of determining best rate of climb.

Bob Gardner

"O. Sami Saydjari" wrote in message
...
In my Cessna 421C, the pilot-side airspeed indicator reads 6 nts below the
co-pilot-side airspeed indicator. When I pointed this out to a mechanic,
he said that it was "within toleraance." He said it was possible that one
was 3 nts high and the other 3 nts low and so both could be within
tolerance. He added that there is no real internal adjustment, so there
is not much he could do about it anyway.

Seems to me that when you are talking about a possible 6nt difference and
given the need for accurately maintain blue-line speed in case of a
single-engine failure, it is important to know that one's airpseed
indicator is accurate. Isn't there a way to judge true accuracy of speed
indicators (just as there is for altimeters)? Should I be satisfied with
my mechanic's answer and move on?

-Sami
N5554G, Cessna 421C

Bob Gardner wrote:

If there is no means of adjustment, the question answers itself. Keep in
mind that the blue line is not sacrosanct...actual Vyse will vary with
weight and density altitude. The vertical speed indicator is a much more
accurate means of determining best rate of climb.

Well, not really. My question is whether their is a good way to measure
the absolute accuracy of a ASI. And my concern is that 6 nts is too far
off when it could make a significant difference in performance in
critical situations, like single engine operation.


Bob Gardner

"O. Sami Saydjari" wrote in message
...

In my Cessna 421C, the pilot-side airspeed indicator reads 6 nts below the
co-pilot-side airspeed indicator. When I pointed this out to a mechanic,
he said that it was "within toleraance." He said it was possible that one
was 3 nts high and the other 3 nts low and so both could be within
tolerance. He added that there is no real internal adjustment, so there
is not much he could do about it anyway.

Seems to me that when you are talking about a possible 6nt difference and
given the need for accurately maintain blue-line speed in case of a
single-engine failure, it is important to know that one's airpseed
indicator is accurate. Isn't there a way to judge true accuracy of speed
indicators (just as there is for altimeters)? Should I be satisfied with
my mechanic's answer and move on?

-Sami
N5554G, Cessna 421C

And my point is that absolute accuracy is not important.

Bob

"O. Sami Saydjari" wrote in message
...
Bob Gardner wrote:

If there is no means of adjustment, the question answers itself. Keep in
mind that the blue line is not sacrosanct...actual Vyse will vary with
weight and density altitude. The vertical speed indicator is a much more
accurate means of determining best rate of climb.

Well, not really. My question is whether their is a good way to measure
the absolute accuracy of a ASI. And my concern is that 6 nts is too far
off when it could make a significant difference in performance in critical
situations, like single engine operation.


Bob Gardner

"O. Sami Saydjari" wrote in message
...

In my Cessna 421C, the pilot-side airspeed indicator reads 6 nts below
the co-pilot-side airspeed indicator. When I pointed this out to a
mechanic, he said that it was "within toleraance." He said it was
possible that one was 3 nts high and the other 3 nts low and so both
could be within tolerance. He added that there is no real internal
adjustment, so there is not much he could do about it anyway.

Seems to me that when you are talking about a possible 6nt difference and
given the need for accurately maintain blue-line speed in case of a
single-engine failure, it is important to know that one's airpseed
indicator is accurate. Isn't there a way to judge true accuracy of speed
indicators (just as there is for altimeters)? Should I be satisfied with
my mechanic's answer and move on?

-Sami
N5554G, Cessna 421C

Compare the reading son the airspeed, altimeter and VS while
pressurized and with the cabin vented at ambient. You could
have a static leak inside the pressure cabin that is letting
pressurized cabin air into the pilot's static line.


--
James H. Macklin
ATP,CFI,A&P

--
The people think the Constitution protects their rights;
But government sees it as an obstacle to be overcome.
some support
http://www.usdoj.gov/olc/secondamendment2.htm
See http://www.fija.org/ more about your rights and duties.


"Bob Gardner" wrote in message
...
| And my point is that absolute accuracy is not important.
|
| Bob
|
| "O. Sami Saydjari" wrote
in message
| ...
| Bob Gardner wrote:
|
| If there is no means of adjustment, the question
answers itself. Keep in
| mind that the blue line is not sacrosanct...actual Vyse
will vary with
| weight and density altitude. The vertical speed
indicator is a much more
| accurate means of determining best rate of climb.
|
| Well, not really. My question is whether their is a
good way to measure
| the absolute accuracy of a ASI. And my concern is that
6 nts is too far
| off when it could make a significant difference in
performance in critical
| situations, like single engine operation.
|
|
| Bob Gardner
|
| "O. Sami Saydjari"
wrote in message
| ...
|
| In my Cessna 421C, the pilot-side airspeed indicator
reads 6 nts below
| the co-pilot-side airspeed indicator. When I pointed
this out to a
| mechanic, he said that it was "within toleraance." He
said it was
| possible that one was 3 nts high and the other 3 nts
low and so both
| could be within tolerance. He added that there is no
real internal
| adjustment, so there is not much he could do about it
anyway.
|
| Seems to me that when you are talking about a possible
6nt difference and
| given the need for accurately maintain blue-line speed
in case of a
| single-engine failure, it is important to know that
one's airpseed
| indicator is accurate. Isn't there a way to judge true
accuracy of speed
| indicators (just as there is for altimeters)? Should I
be satisfied with
| my mechanic's answer and move on?
|
| -Sami
| N5554G, Cessna 421C
|
|
|

O. Sami Saydjari wrote:

/snip/Isn't there a way to judge true accuracy of
speed indicators (just as there is for altimeters)? Should I be
satisfied with my mechanic's answer and move on?

-Sami
N5554G, Cessna 421C

Sami,

There certainly are maintenance actions you can take to ensure your
airspeed indications are as accurate as possible. Both airspeed
indicators can be sent to an instrument shop for check/overhaul. Any
competent mechanic can check your pitot and static systems for leaks and
/or abnormalities. Once these are all done, your system will be as
accurate as originally designed. Keep in mind that *some* aircraft are
designed so that one side is accurate at lower speeds, and the other
accurate at higher speeds, but I doubt this is the case with a twin
Cessna. The above info does not infer whether the expense incurred
would be worth the outcome, that is a decision only you can make.

P.S. When did you sell your Turbo Arrow and get that bottomless money pit?

Happy Flying!
Scott Skylane

On Sat, 07 Jan 2006 09:34:26 -0600, "O. Sami Saydjari"
wrote:

In my Cessna 421C, the pilot-side airspeed indicator reads 6 nts below
the co-pilot-side airspeed indicator. When I pointed this out to a
mechanic, he said that it was "within toleraance." He said it was
possible that one was 3 nts high and the other 3 nts low and so both
could be within tolerance. He added that there is no real internal
adjustment, so there is not much he could do about it anyway.

Seems to me that when you are talking about a possible 6nt difference
and given the need for accurately maintain blue-line speed in case of a
single-engine failure, it is important to know that one's airpseed
indicator is accurate. Isn't there a way to judge true accuracy of
speed indicators (just as there is for altimeters)? Should I be
satisfied with my mechanic's answer and move on?

-Sami
N5554G, Cessna 421C

There are units, similar to those used for altimeter checks, that can be
used to verify ASI accuracy on the ground.

In addition, if you can perform a TAS calculation, and you have a GPS on
board, you could do three runs at 90° angles to each other, and compare
your real TAS to the one obtained from your ASI -- of course that only
gives you the error at one point, but that might be enough. (There are
tools on the web to enable you to calculate your TAS from the three GPS
groundspeed determinations).




Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

I wrote an article in Kitplanes magazine 15 years ago or so about using
plastic tubing, a yardstick, and tinted water to make a water manometer for
testing airspeed indicators.



"O. Sami Saydjari" wrote in message
...
Bob Gardner wrote:

Well, not really. My question is whether their is a good way to measure
the absolute accuracy of a ASI. And my concern is that 6 nts is too far
off when it could make a significant difference in performance in critical
situations, like single engine operation.

Scott Skylane wrote:
O. Sami Saydjari wrote:

/snip/Isn't there a way to judge true accuracy of

speed indicators (just as there is for altimeters)? Should I be
satisfied with my mechanic's answer and move on?

-Sami
N5554G, Cessna 421C


Sami,

There certainly are maintenance actions you can take to ensure your
airspeed indications are as accurate as possible. Both airspeed
indicators can be sent to an instrument shop for check/overhaul.

Arg. Sounds like a hard way of finding out.

Any
competent mechanic can check your pitot and static systems for leaks and
/or abnormalities.

Just did a Pitot Static check.

Once these are all done, your system will be as
accurate as originally designed. Keep in mind that *some* aircraft are
designed so that one side is accurate at lower speeds, and the other
accurate at higher speeds, but I doubt this is the case with a twin
Cessna. The above info does not infer whether the expense incurred
would be worth the outcome, that is a decision only you can make.

P.S. When did you sell your Turbo Arrow

I have not sold my beautiful Arrow yet.

and get that bottomless money pit?

I got half-share in the C421 in mid-November. So, the good news is that
it is only a half-bottomless money pit


Happy Flying!
Scott Skylane

Don't know if this helps. It is important to remember that the radial blue
line is based on sea level, on a standard day, at max gross weight. Change
any of the parameters and the blue line is inaccurate.

Bob Gardner


§ 23.1323 Airspeed indicating system.
(a) Each airspeed indicating instrument must be calibrated to indicate true
airspeed (at sea level with a standard atmosphere) with a minimum
practicable instrument calibration error when the corresponding pitot and
static pressures are applied.
(b) Each airspeed system must be calibrated in flight to determine the
system error. The system error, including position error, but excluding the
airspeed indicator instrument calibration error, may not exceed three
percent of the calibrated airspeed or five knots, whichever is greater,
throughout the following speed ranges:
(1) 1.3 VS1 to VMO/MMO or VNE, whichever is appropriate with flaps
retracted.
(2) 1.3 VS1 to VFE with flaps extended.
(c) The design and installation of each airspeed indicating system must
provide positive drainage of moisture from the pitot static plumbing.
(d) If certification for instrument flight rules or flight in icing
conditions is requested, each airspeed system must have a heated pitot tube
or an equivalent means of preventing malfunction due to icing.
(e) In addition, for commuter category airplanes, the airspeed indicating
system must be calibrated to determine the system error during the
accelerate/takeoff ground run. The ground run calibration must be obtained
between 0.8 of the minimum value of V1, and 1.2 times the maximum value of
V1 considering the approved ranges of altitude and weight. The ground run
calibration must be determined assuming an engine failure at the minimum
value of V1.
(f) For commuter category airplanes, where duplicate airspeed indicators are
required, their respective pitot tubes must be far enough apart to avoid
damage to both tubes in a collision with a bird.

[Amdt. 23-20, 42 FR 36968, July 18, 1977, as amended by Amdt. 23-34, 52 FR
1834, Jan. 15, 1987; 52 FR 34745, Sept. 14, 1987; Amdt. 23-42, 56 FR 354,
Jan. 3, 1991; Amdt. 23-49, 61 FR 5168, Feb. 9, 1996]

"O. Sami Saydjari" wrote in message
...
Scott Skylane wrote:
O. Sami Saydjari wrote:

/snip/Isn't there a way to judge true accuracy of

speed indicators (just as there is for altimeters)? Should I be
satisfied with my mechanic's answer and move on?

-Sami
N5554G, Cessna 421C


Sami,

There certainly are maintenance actions you can take to ensure your
airspeed indications are as accurate as possible. Both airspeed
indicators can be sent to an instrument shop for check/overhaul.

Arg. Sounds like a hard way of finding out.

Any competent mechanic can check your pitot and static systems for leaks
and /or abnormalities.

Just did a Pitot Static check.

Once these are all done, your system will be as accurate as originally
designed. Keep in mind that *some* aircraft are designed so that one
side is accurate at lower speeds, and the other accurate at higher
speeds, but I doubt this is the case with a twin Cessna. The above info
does not infer whether the expense incurred would be worth the outcome,
that is a decision only you can make.

P.S. When did you sell your Turbo Arrow

I have not sold my beautiful Arrow yet.

and get that bottomless money pit?

I got half-share in the C421 in mid-November. So, the good news is that
it is only a half-bottomless money pit


Happy Flying!
Scott Skylane