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Old 16th Sep 2017, 18:36   #1 (permalink)
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Join Date: Nov 2015
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Question on Continued Airworthiness

This relates to a couple of Cessna aircraft I have flown that had Robertston STOL installations.

On application of 10 degrees or greater flap, the aircraft rolls left and will continue to roll inverted if the pilot does not intervene. The force required to roll wings level and hold them there is quite noticeable. It has to be held until landing.

The roll instability is there on takeoff as these aircraft call for 10 flaps on takeoff, but while it is noticeable once the wheels lift off, it does not require as much force at lower speeds. Same on approach.

It seems to me that the certificate of airworthiness is invalid when this situation exists as the aircraft does not meet the standards under which it was certified. Is that a correct assumption?
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Old 18th Sep 2017, 11:59   #2 (permalink)
 
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On the face of it the aircraft the aircraft need a full rigging check,to basic datums ,and then flight checks by a qualified test pilot.....do not fly them in this condition,they are not fit for service,and the insurance may also be invalid.
`Step Turn` ,or` Pilot DAR`would be best to contact for advice as well.....
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Old 19th Sep 2017, 11:39   #3 (permalink)
 
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Trying to think this through as an EASA 21J HOA and CVE. The behaviour you describe doesn't sound acceptable to this low time PPL, so is the system correctly implemented, set-up and maintained? (Lesson learnt long ago as a junior development engineer, when someone tells you the design is wrong, first check that they made it to the design). There must be a STC for the modification kit (well, assuming that the aircraft is not registered Experimental and that its type certificate and CofA are still valid) so the first thing is to check that you have a legal modification and that it is correctly implemented. A quick search on the FAA website suggests that FAA STCs can be found via the link below. Look under By STC Holder for "Sierra Industries, Inc, Garner Field Road Uvalde TX 78801 United States". There is a long list, you'll have to find the one or more that apply, it should be recorded in the aircraft paperwork.


http://rgl.faa.gov/Regulatory_and_Gu...e?OpenFrameSet


That should get you started.
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Old 19th Sep 2017, 14:39   #4 (permalink)
 
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Quote:
Originally Posted by cncpc View Post
This relates to a couple of Cessna aircraft I have flown that had Robertston STOL installations.

On application of 10 degrees or greater flap, the aircraft rolls left and will continue to roll inverted if the pilot does not intervene. The force required to roll wings level and hold them there is quite noticeable. It has to be held until landing.

The roll instability is there on takeoff as these aircraft call for 10 flaps on takeoff, but while it is noticeable once the wheels lift off, it does not require as much force at lower speeds. Same on approach.

It seems to me that the certificate of airworthiness is invalid when this situation exists as the aircraft does not meet the standards under which it was certified. Is that a correct assumption?
Unfortunately Sycamore is missing something potentially important - Pilot_DAR was unfortunately in a seaplane crash a little while ago and is mostly out of circulation still, so he's probably not the right man to ask for help right at this minute. If I was in Canada, I'd gladly help, but I'm not.


Sounds to me that what you are suffering from here is not necessarily a roll instability - sounds to me more like an undemanded roll input. If this is *only* happening at a particular flap setting I'd be suspicious particularly of the flap mechanisation on the particular aeroplane, rather than the STOL kit. Really obvious things to check would be the actual flap position on each side through range - easily checked using a suitable phone app to an adequate standard for a quick and dirty look at this.

If it is the result of the STOL kit, if the kit is STCd - which means it's presumably been flight tested by a grown up in some previous installation and found to be satisfactory - then the other likely culprit is that the STOL kit (vortex generators IIRC) have been installed incorrectly.

Either way, I think that your first port of call is to get the aeroplane checked over for rigging and correct installation of the STC by your friendly neighbourhood licenced engineer / A&P.

G
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Old 27th Sep 2017, 02:17   #5 (permalink)
 
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Quote:
Originally Posted by cncpc View Post
....

It seems to me that the certificate of airworthiness is invalid when this situation exists as the aircraft does not meet the standards under which it was certified. Is that a correct assumption?
I would agree that flap rigging or similar seems a possible culprit.

But with regard to the question above, I believe the answer is "no". The C of A is not invalid.

This is because whether an aircraft is 'airworthy' and whether it has a valid CofA are not the same thing. A valid CofA simply requires that the conditions attached to that are followed - the requisite required mainyenance has been performed, per the servicing intervals laid down in the ICA (Instructions for Continued Airworthiness). It is quite possible that this aircraft is not airworthy - it has an unresolved defect which appears quite serious. But the C of A can still be valid.

Consider: mx inspects the flap rigging, makes adjustments, and asks for a check flight to confirm its fixed. Can you do the flight? - sure. Did you need to re-apply for a new CofA? - no.

Not only can a non-airworthy and even 'unsafe' a/c have a valid CofA but an actually safe and airworthy aircraft can have an invalid CofA if you didn't follow the paperwork correctly. Thats not to say the two concepts are totally disassociated, but they are not 100% linked either.
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Old 7th Oct 2017, 16:31   #6 (permalink)
 
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You will find details about a lot of the installations in the CAA AAN lists, I have included them below for the Roberston STOL

AAN Search Results Summary
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Old 9th Oct 2017, 18:13   #7 (permalink)
 
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I have owned several Cessna aircraft that had the Robertson STOL system installed including the C182, 206, 337, 421B and 421C I have also owned them as standard aircraft.

I was approved by the CAA to carryout flight tests on all these aircraft. Mainly at max gross and aft C of G.

They are interesting aircraft to fly as the modifications do enable you to take off, approach and land at significantly lower speeds but some can lull you into a false sense of security in that some have a lower angle of attack than the norm but at a much lower speeds, so be wary in making a steep turn or orbit on final.


The leading edges on some are recontoured by means of a cuff. Wing fences rather than VGs are installed. The flaps are interconnected to the ailerons as they droop when 10deg of flaps is selected therefore the flap and aileron rigging and tension are critical and you need to refer to the rigging procedure shown in the Service Manual supplement that should be with the aircraft.

A ground check will show if the flaps extend to the same degree and at the same time. When extended check the flap guides and rollers for any play, as G says use the app on your phone. Try this at every setting as the ailerons return to near normal as the flaps extend further.

As your C of A should still be valid any flight tests should be carried out at altitude.

Fly with someone you trust and who knows the aircraft and write a proper flight test schedule covering all aspects pre-during and after flight. Choose a midpoint C of G balanced fuel load. You can take off with zero flap, you can also land with zero flap as well but they are different. Identify the flap motor CB. Climb to safe altitude and be sure that you keep the ball in the centre and reduce the speed to 20kt less than the max flap speed and SLOWLY extend the flaps and see what happens don't just select the take off setting in one go.

Don't test aircraft near the ground and don't do anything that you are not entirely happy in doing and do involve your engineer in what you are doing. Enjoy.
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