Pilot_DAR wrote:
If you install the OEM kit in accordance with the parts catalog, and the flight manual contains the required information already, you’re set, it’s not a modification (the data is there already).
The flight manual says:
In case of alternate static source utilization, open air outlets and/or actuate cabin air selector flow lever to open position. Then, airspeed indicator and altimeter errors are not significant.
This is exactly why I said it’s not a modification.
Ultranomad wrote:
This is exactly why I said it’s not a modification.
Yes, as long as it is the factory alternate static kit, that flight manual reference is what you need.
Static ports are really fussy things. I’ve done a lot of flight testing to prove them, both external, and a few internal alternate ports. I have seen some major errors, in both airplanes and helicopters, right up to the DC-3. As long as you know it’s in accordance with the factory data, or other approved data, you’re fine. If you’re not sure, it’s worth understanding…
@Pilot_DAR, by ‘major errors’, did you mean patently improper designs or merely large corrections? Please tell more, it’s a very interesting subject.
I’ve had occasion to test fly for design approval, aircraft which have either had a modification which directly affected the static port(s), or mods where the changed performance of the modified airplane took it into a speed range for which no position error correction data was published, and there was a large position error:
My client installed a survey boom on an AS350 helicopter. The installation affected the center belly mounted static ports. One of my jobs was to “find” a new static port, as the error produced by the effect of the nearby boom attachment was in excess of 30 knots. I experimented with temporary static ports at various locations around the cabin, none were good. I did 6 hours of expensive flight testing, with no potential good location. As I now did have a survey boom, which projected forward beyond the rotor disc, I drilled four holes (top & bottom, side & side) at the nose of the boom. I connected the four together, and that connection became the new primary static port. It did require moving the static line to the boom each time it was installed, but it was otherwise an ideal solution. I certified it that way. It was later found that some installers were wrongly making only one static port in the nose of the boom, and there were large airspeed errors again. I heard that the fleet with these booms were grounded, until the correct static port arrangement was installed (which was not a big job!).
Another AS350 modification (with which I was not originally involved) had an under fuselage installation. An in flight “event” caused me to suggest to the operator that there was probably an unknown static port (position error) problem. The original designer of that under fuselge mod told me that he had flown position error testing using the GPS method. However, that method requires powered level flight, and the error I suspected was in autorotation. I installed my swivelling pitot head temporarily for position error testing. I flew the testing, and found a previously unknown position error of 12 knots on the slow side during autorotation (quite dangerous – like gliding 12 knots slower than best glide speed in an SEP). The operator permanently withdrew the modification from use.
One of my clients installed bubble cabin windows. The windows disturbed the air just enough to introduce a detectable position error. I flight tested for it, and created a new position error chart – it was within certification limits, just a little different.
Another client installed an Alpha Systems AoA in a modified Cessna 206H. The airplane had STC’d VG’s and wing extensions – it had a silly slow stall speed! I was asked to approve the installation. I went to flight test the airplane to do the initial set up of the AoA with the Avionics tech. I followed the OEM installation and set up instructions. This centered upon bringing the airplane to the point of the stall, recording that speed, multiplying it by 1.3, flying that, and making that the set point for the AoA. The result was 47 KIAS approach speed. Flying a power off approach in a 206H at 47 KIAS is terrifying! (even the non pilot Avionics Tech was squirming!). What I found upon more testing was that an airborne __indicated___Underline_ airspeed of 36 knots was possible. That could not be right for a 3600 pound plane! So, after some buggering around, I reset the AoA set point speed to exactly the Cessna approach speed. The client was unhappy as this did not enhance the “STOL” of the airplane. I was unwilling to lure him into that regime with poor airspeed indication. What it turned up was that the AoA installation instructions should have said to set the AoA set point to 1.3 of the stall speed in CAS, rather than IAS. that would have flagged that there is no position error correction for the Cessna 206H which goes down to 36 knots IAS. So accurate set up of the AoA on that airplane is not possible using the data available. Very few SEP mods come with new position error correction tables. And, the few that do, certainly do not come with the table for combinations with other mods! The CAS to IAS difference for that C 206H at 36 KIAS, is around 17 knots, so the actual stall speed is 53 KIAS. Sure, you can “find” a comfortable appraoch speed to fly, but if you’re actually factoring a published or indicated airpseed, knowing IAS to CAS becomes pretty important.
My much loved Horton STOL C 150M would fly steadily at 22 KIAS (I installed a Bell 206 airspeed indicator, as I could zero the pointer of the Cessna ASI). But when I installed my swivelling pitot head on the 150, the actual stall speed was 41 MPH CAS – much more turthful!
While flight testing a Cessna 210G for other purposes, I noticed that the static port is close behind the main landing gear doors. Hmmm, when those doors open, there must be quite a disturbance. I looked, during gear cycling – yes, the airspeed jumped around, and the altitude jumped 200 feet momentarily. But it was transient only with gear transit, and certified that way, so I did not get any further involved.
Those are a few. I’ve also done PEC testing on Partinavia, C172, MD500, Caravan, and DC-3T. There’s a lot to be learned about this! If you’re going to depend upon the ASI, best that the pitot static system conform to approved data!
Following up on the matter, the EASA avionics shop I was talking to seems bit “hesitant” in regard to used parts without a Form 1 – is there a way to make that part legal?
My assumption was they could just identify the part given the illustrated parts catalogue and issuse a Form 1 themself? (Lable it as “repaired/inspected” or similar?)
slider wrote:
My assumption was they could just identify the part given the illustrated parts catalogue and issuse a Form 1 themself? (Lable it as “repaired/inspected” or similar?)
To inspect/test and release a used component on a Form 1 requires that the maintenance company holds current approved maintenance data for that item and it’s listed on their Part 145 capability list under a C rating. Not much chance of that in this situation.
