Funny you mention this being an Arrow. It was the Arrow that I used to fly which showed a low IAS, until we found out that it was indeed a leaky airspeed indicator. Here it is, showing about 12 knots less than it should indicate:
Regarding repainting: you should pretty much abandon your hopes on that. Other people who repainted their aircraft found out that the speed increase was exactly: zero.
jgmusic wrote:
engineering tells me the age of the paintwork is the biggest culprit
I found that a wash, and a good coat of wax to an otherwise decent paintjob is worth a few knots.
My experience with Pipers has been different than that of Cessnas in this respect:
I have found Cessna POH’s to be pretty accurate, provided you fly in the stated configuration (wheel fairings for one thing). This seems to be because Cessna uses their POH as the “Flight Manual” for the plane, which is a requirement for certification.
While performance testing a Piper twin Comanche years back, I could not come close to the Piper POH values, and was puzzled. This plane was in beautiful condition, and if any could match the POH, this one should. Like the wisdom of Yoda, my test pilot mentor quietly told me to refer to the FAA approved flight manual for the Twin Comanche. Not thinking, I said I had. “No, you had not…” came the wise words… Al was right, the Twin Comanche, (and I later learned the Navajo too) had both a fancy Piper POH, and an FAA approved flight manual. The flight manual was very minimalist, providing only the information required by the rules to be provided – no cruise performance information. This left the POH as an entirely non approved (verified) document, probably originating from the marketing department, rather than the flight test department via the FAA. Thus, the Piper POH performance figures could say whatever they wanted, and there was no authoritative check of their accuracy.
It’s been a while since I’ve have the pleasure of flying an Arrow (I really like them), so I have not reviewed their POH/flight manual relationship. But have a look, if there is one of each for the plane, and the flight manual is identified as FAA approved, that means that the POH is certainly not FAA approved, and perhaps contains the optimism of a marketing department. In the certification basis of an Arrow, there is no requirement for the manufacturer to provide cruise performance data at all. The fact that Cessna provides cruise performance data throughout the legacy of their airplanes is just Cessna being thorough. When I have checked the Cessna performance values in conforming airplanes, I have found them to be pretty accurate. The POH climb performance values for the Twin Comanche were at least 15% very optimistic.
One thing to check is the accuracy of your instruments, in particular the tach for power and the ASI for airspeed. I once owned a C150 and leased it to a flight school. The ASI indicated lower than the book, while another C150 indicated much higher than book. Students and instructors preferred the other C150, but wing tip to wing tip at full throttle cruise, they were identical, but the other C150 claimed it was 20 MPH faster. If you want to indicate more speed, create a static leak, that will be good for at least 5 kts or more. :)
I have found that in both the Navajo and the Chieftain I can better book figures, both in speed and fuel flow, though that may be a question of rarely being at MTOM and having better engine instruments.
Here is a small footage of the 1985 turbo arrow I’m flying; FL80@65% (2400/30), 120kias which does 139Ktas (0°C@FL100), but had some headwind. This is inline with the Poh actually and engine is not really out of overall…
Although I admit the Asi is a bit old, and takes some time to indicate good airspeed, especially at takeoff.
boscomantico wrote:
until we found out that it was indeed a leaky airspeed indicator
Funnily enough, an ASI leak was detected in my recent annual but it came back showing exactly the same speed after repair 
A drawback of re-doing our PA28-161 interior is we changed all the door gaskets. Water does not get into the plane anymore, but the new gaskets are very stiff and push said doors outwards. After many ins and outs the main door is now flush to the fuselage, but the baggage compartment door probably sticks out 5-10mm, which can’t be good for aerodynamics. On the other hand re-doing the exterior means the few dings and bends on the wings and control surfaces have been addressed, and we probably gained there vs before the overhaul. Also, the number of antennas on the roof is now twice that of what it was when the plane came out of the factory 38 years ago. All in all we are within 1-2 knots of book values.
I have not tried the wax, but I have been told bugs is worth up to 2 knots.
The Arrow II I flew for the CB-IR produced within 2-3 KTAS of book values, which is fine for a 50 year old bird.
Anyone here remembers what is the calibration limit for the ASI?
greg_mp wrote:
Here is a small footage of the 1985 turbo arrow I’m flying
Ok, so my 125Kts IAS would seem right for a non-turbo Arrow IV? I thought it might have reached higher but, as Pilot_DAR says, there’s a lot of smoke and mirrors with marketing claims… I’ll stop worrying about it. Thanks for all the help and interesting info.
I would say as suggested, take a note of airspeed at some point on the air, and cross check with poh. Usually only tas is shown but you can compute it on the ground, or take a computer with you.
The TB10 POH states that in ISA conditions at 6,000ft with 22.1" MP and 2400 rpm I should be seeing 112 KIAS.
This is wildly optimistic.
In those conditions and at that altitude, I will see about 105-106 KIAS with 23.5" MP (wide-open throttle) and 2400 rpm.
I don’t think it is instrument error. I monitor ground speed closely and would notice if was an there was unexplained discrepancy between ground speed, the forecast wind, and KIAS/KTAS.
I posted the other week about my confusion over the POH, why it does not seem to show WOT MP values for cruise power settings at 6,000 – 10,000ft and why I thought it might contain errors of some sort.
