. was in an unfortunate situation, with a TC Inspector, who, for what ever reason saw a different circumstance. I agree that TC inspectors are human, and entitled to be wrong every now and then. I guess that every now and then, a person digs in to being wrong, and it just gets worse. That's why the TC system has "other people" to speak to when there seems to be an impasse.
. is correctly able to articulate:
he would not accept the Tail wheel Aerobat because it was not a Cessna 150 anymore.
I said have you checked the C of A and read the STC that was part of it.
A certified aircraft, when modified by STC is still
that model aircraft. Indeed, it would be very hard to modify a certified aircraft that it became a certified aircraft of a different model by C of A. I would venture that by the time someone accomplished this, the aircraft would have cost too much to make it available for flight training! But, it is the owner/applicant's responsibility to understand what aircraft they are applying with, and what it's privileges are.
It's pretty common now for legacy aircraft to have STC'd mods. Of course, "STC" stands for "Supplemental Type Certificate". The original aircraft was certified in accordance with a Type Certificate, so now it's supplemented. That supplement will be done to either match, or be to a newer (higher) standard than the original certification, so there is no reason to think that the aircraft is
less compliant with the mod installed, with
three important exceptions:
Restricted;
In a few cases, an STC could introduce a restriction, in which case, the word "Restricted" will be prominent on the STC certificate. This will restrict how the aircraft is to be used. Most commonly - no passengers. Examples of this would be research/SAR/firefighting aircraft, where there is no intention to carry pax anyway. Somewhere in the STC "demonstration of compliance" process, it will have agreed that the aircraft no longer meets the requirements for passenger safety, but that's still okay for the intended crew only role. An example might be that a cabin exit has been blocked, or cabin flammability requirements are no longer met.
Limitation;
The STC might introduce a limitation which affects it's operation with the STC mod installed. The aircraft is still otherwise compliant, but something it used to be allowed to do, is no longer permitted. This will not affect passenger carrying, but will affect something else. The only "Cessna" type example which comes to mind is that the STC installation of a Horton STOL kit will remover the permission to intentionally spin 150/152/172. This is not because the aircraft is less able to spin and recover (it still meets the requirement to be recoverable), but rather, it can no longer be held in the required number of turns, and then recovered - the spin changes/recovers itself before that requirement is demonstrated. A very safe characteristic, but non ideal for teaching spins! You can still train in these aircraft - just not spins - it's a limitation.
And, "Frankenplanes";
My personal record is a wonderful 182 amphibian, which in addition to a few unique mods I did to it, has 27 off the shelf STC mods installed. Is it still a 182Q? I don't think Cessna would think so, but it still has the Cessna data plate, with the type certificate number, and a C of A saying that's valid. But the aircraft is so highly modified that many speeds, weights, dimensions, systems, and limitations are not what the aircraft left the factory as being. The result was a lot of document writing on my part, to define how to operate and maintain this highly modified aircraft. I got to that step before the maintenance person did, because I was involved from the beginning of the modifying. However, for aircraft which go into the shop for "one more" mod, the installer/maintainer is responsible for considering the following condition which is on every STC:
Conditions: This approval is only applicable to the type/model of aeronautical product specified therein. Prior to incorporating this modification, the installer shall establish that the interrelationship between this change and any other modification(s) incorporated will not adversely affect the airworthiness of the modified product
That's a big one, and worthy of the thought. Either of the two STC's could be well compliant, but combine them, and the aircraft becomes non compliant - because each STC holder did not consider (and test) in combination with the other STC'd modification (there's no expectation that they would - hence the statement). The installer now responsible for discharging a certification expectation, which could be well outside their base of knowledge and experience. Often, it requires another STC to allow the combination of the previous mods, and limitations could be imposed. An example of this which I was involved in testing back in the late '80's is the combination of a carburetted 520/550 in place of the O-470 on a Cessna 180/182 + a STOL kit. Either is a fine change to the aircraft, and one would think that both together are better yet. However, the gravity feed fuel flow requirement is greater for the 520/550 than it was for the O-470, and the airframe still meets this requirement for the required "attitude least favourable" - with a normal airfoil. But, modify the airfoil so the aircraft can fly at a greater yet AoA, and you may find that the carb can be higher than the fuel level with less than 1/4 tanks, fuel stops flowing, and the engine quits with the nose pointed very high. I have experienced this during flight testing - it would have been really startling, if it were not what I was actually trying to accomplish in the test.
So, it's up to
you, the owner/applicant to understand ALL of the effects of the mods you choose for your aircraft. There are many fine mods available by STC, and each will benefit the aircraft somehow. Just assure that you also understand any unintended changes which could accompany the modification to the aircraft, and how that could affect your intended use of the aircraft.
Oh, by the way, the aircraft in my little avatar picture was modified to the point where STC approval was not possible (compliance could not be demonstrated). The only way to continue to fly the aircraft at all, was that it be accepted into the Owner Maintenance category. TC agreed to allow that, so I bought the plane, and moved it over. That was an example of an eager person, who (albeit with an FAA DER Engineer leading the way) put too many changes, and the result was not certifiable. Without OM, that plane would never have flown again.
