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  #1  
Old 06-04-2021, 08:50 AM
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WA85 WA85 is offline
 
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Default Miserable Quality Loves Company - Part 2 Safety Margins

I am hi-jacking Captain Avgas's thread and trying to re-focus the conversation to the topic I think everyone is trying to address - avoiding a catastrophic accident resulting in the death of one of our fellow RV pilots.

Disclaimer - I built a very imperfect RV-8 and I have several of the well known quality issues / errors that I wished I would have learned about when I was building my RV-8 (firewall penetration's with poor fire-proofing, my canopy has 4 cracks in it, I didn't prime the a few parts, my avionics wiring isn't perfect) and I can't claim to be a perfect mechanic either.

However I think what many (Vic, Dan, Walt, Bob, Sam) on this forum are trying achieve, is to point out repetitive quality issues that can lead to a catastrophic outcome.

Instead of trying to define what is acceptable "quality", maybe another way to look at this issue is how does the "defect / quality issue", affect the safety margin and the consequences of the defect / quality issue, which could lead to a catastrophic outcome.

There FAA uses FARs (i.e. 23, 25, 27, 29 etc) to try to encourage aircraft manufacture's to design and build aircraft with known factors of safety and safety margins for safe operation of aircraft.

FAA TSO's are another form of known factors of safety and safety margins for safe operation of avionics.

The venerable FAA AC43-13 is a form of known factors of safety and safety margins for inspection and repair of aircraft and systems.

The DoD has Mil-Hdbk-517 which is used to establish equivalent levels of known factors of safety and safety margins for safe operation of aircraft.

The FAA uses a myriad of processes to establish and maintain safety margins for the inspection, maintenance and repair of "certified aircraft".
A&Ps, IA's, Part 145 R-P are supposed to be trained to follow safe maintenance process and use approved parts and procedures.

Often times we see fellow forum members pointing to these very same requirements as a basis for they think is "acceptable quality".

In the EAB world, very few of these "safety margins / requirements" exist, but that is the nature of beast and some people love the freedom to do that they want.

There are routine examples of "doing what we want" in this forum. Some are brilliant ideas, some are foolish.

Who determines what is brilliant or foolish, this seems to be where the emotional rub seems to be.

Those of use who have careers in aviation safety / airworthiness / engineering see things on this thread that scare us and as professionals, we speak up, often to severe criticism.

Maybe a better way to for the aviation safety / airworthiness / engineering experts on the forum to express their concerns is to explain the loss of "safety margin" and the potential outcomes.

One of the great popcorn topics is installing the GPS / navigator antenna under the engine cowling.

Yes, it will and does work.

However, if you read the fine print for the installation of GPS antenna, it does not approve the installation of he GPS antenna in the operating environment under the engine cowling.

Yes, it will work, but have you looked at the loss of safety margin that you created by doing this?

Did you do environmental testing above and beyond what Garmin has done?

Its likely you copied someone else's installation found on the interweb.

During subsequent real world IFR operations in the NAS, you are now betting your life, and those of others, (especially if you are RNAV only) by your decision to install the GPS antenna in an unapproved location...but it works...until it doesn't and you become and accident.

Why is installing the GPS antenna in an unapproved environment a safety issue? If you think about it, that GPS antenna may be the sole piece of equipment that is providing differential GPS signal to provide the required performance to you navigator / auto-pilot to fly an LPV approach to minimums. It also might be the sole position source for your ADSB out data.

Both are safety critical functions, with a single point of failure at the GPS antenna, that could lead to a catastrophic outcome (erroneous GPS position leads to splat with the ground or another aircraft).

There are host of other topics to debate, but maybe the best way to work through these is issue is for every builder and owner operator to ask themselves if their maintenance, inspection or repair actions reduce the safety margins of their aircraft and how they will possibly affect a catastrophic outcome to themselves and others.

Okay , I am out of popcorn.

As a side note, I have found several issue on my own aircraft that I think lack sufficient safety margin and I am working to resolve them. My firewall penetrations are lousy, I need to fire proof them. My canopy has too may cracks now, its structural integrity is questionable...I have new canopy in the works.
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  #2  
Old 06-04-2021, 09:45 AM
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airguy airguy is offline
 
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Your point is valid - but I would submit the question to the audience about how many times has anyone seen a performance-related issue that may be attributable to a FWF antenna location? It's most likely that the antenna mfr does not recommend a FWF location simply because it was never tested - not because there was any indication of a problem.

I've got 850 hours on my installation (x3 antennae) with no failures and no RAIM warnings - just one data point.

It's only a problem, if it's a problem. Otherwise it's a solution in search of a problem.
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  #3  
Old 06-04-2021, 10:02 AM
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rv8ch rv8ch is offline
 
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Great initiative!

Here's my $0.02 - worth exactly what you paid for it.

As someone who hiked down the "I know better than Van" path and had to do a long and expensive walk of shame back to the well-traveled path, I would recommend to every builder to carefully read the plans and the manual, and do it as close as possible to the factory recommendations on their first aircraft, and to listen to the wise advice of the "greybeards" at Van's, on VAF, and in your local chapter.

After you have the experience of doing it like the plans say, and you want to do something truly experimental, do it on your second airplane!
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  #4  
Old 06-04-2021, 11:24 AM
terrye terrye is offline
 
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Default Miserable Quality Loves Company - Part 2 Safety Margins

Interesting approach thinking about safety margins. I'm still building and one of the considerations is where to put the GPS antennae. I'm trying to decide whether to put one or two under the cowling, or one or two on the aft fuselage next to the canopy slider rail, or one under the cowling and one on the aft fuselage.

One of the problems with 2 seat RV aircraft is their small size compared to certified aircraft, so the placement of antennae is a problem, especially with lots of antennae (GPS, XPDR, COM, ADSB, ELT, Stormscope, etc.). Each has its footprint, and most recommend a minimum spacing from other antennae. Another is aesthetics, although this should be secondary to function.

So how should I make this decision for GPS antennae? Garmin has tested positions on the fuselage (forward of the windshield, aft fuselage) and tested them in temperature and environmental conditions as well as for signal masking for general use. But not on RV aircraft! If you position the antennae too far aft (say on an RV-8) due to canopy travel, is the antenna masked by the proximity to the vertical stabilizer? If you position two of them side by side on the aft fuselage on either side of the canopy slide rail are they too close to each other and therefore give erroneous signals to the navigator?

On the other hand, many many GPS antennae have been tested by owners/builders/pilots under the cowling and reported good results over long periods of time. Except for the environmental aspects of heat (which could be mitigated by blast tubes) this seems like the perfect position. Good view of the sky, minimal masking by structure, with the proviso not to use metallic paint on the cowling. So here's an example of an actual installation on an RV aircraft, extensively tested by a large group of pilots under actual operating conditions, with few to none failures reported due to being mounted in a relatively high heat environment. Is the under cowl heat environment in an RV worse than a top of fuselage mount on a certified aircraft operating in Central America or near the equator in Africa? Don't know.

How much and what kind of testing of GPS antennae under the cowling of RV aircraft is sufficient to provide the same safety margin as Garmin's recommendations? Supposing that Garmin's recommendations have a safety margin of 1.0, what is the safety margin of mounting under the cowling? Or conversely, how much is that safety margin of 1.0 degraded by the proximity to the vertical stabilizer, or having two antenna mounted on the top fuselage but within 12" of each other because further apart means the antenna is mounted on a curved part of the fuselage?

I've limited my comments to GPS antennae since that is a problem I'm wrestling with. But the OP's point is really about how much testing is adequate to ensure an adequate safety margin and is well taken. It's easy for primary structure where the FARs provide lots of guidance. It's not so easy for systems and especially for system to system interactions.
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  #5  
Old 06-04-2021, 12:22 PM
spatsch spatsch is offline
 
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That actually makes sense. Focusing on safety margins is exactly what one should do.

E.g. in the GPS example. I fly VFR only and look outside while I do so. So for me a GPS failure is a low risk issue. Rather an inconvenience than a safety issue. So I don't obsess about it (actually put mine in the wing tip as its the only place I get acceptable coverage inverted to pass ADS-B. Under cowling the engine blocks the signal inverted... ). If I flew hard IFR I would think a lot more about it.

Similarly with a screwed up rivet. Might not be up to FAA standard but Alcoa says they get a large fraction of their strength and if it is one in a particular area the reduction in margin is low. So in that case the important question is not is the rivet screwed up or not but where is it and how many others are screwed up too.

If people would comment using that approach you would not only get more adoption but we would learn more from each other too.

This of course requires the commentator to understand the actual problem not just be able to cite some regulation ... . I have the feeling that there are many long term experts around that can't explain why they are doing what they are doing beyond because the FAA says so.

Oliver
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  #6  
Old 06-04-2021, 01:07 PM
PhatRV PhatRV is offline
 
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Quote:
Originally Posted by rv8ch View Post
Great initiative!

Here's my $0.02 - worth exactly what you paid for it.

As someone who hiked down the "I know better than Van" path and had to do a long and expensive walk of shame back to the well-traveled path, I would recommend to every builder to carefully read the plans and the manual, and do it as close as possible to the factory recommendations on their first aircraft, and to listen to the wise advice of the "greybeards" at Van's, on VAF, and in your local chapter.

After you have the experience of doing it like the plans say, and you want to do something truly experimental, do it on your second airplane!
I am adding my 0.02 regarding the Vans FWF plans . I think the RV FWFs vary a lot from one airplane to the next is the Vans FWF plan is all over the map. Even when the use of the fuel flow transducer is prevalent with all the newer build, Vans still hasn't update the plans to include it, maybe except for the newer RV14 kit. BTW this is where the builder gets into problem interpreting into things that are not well documented in the Vans plan.
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  #7  
Old 06-04-2021, 01:20 PM
wawrzynskivp wawrzynskivp is offline
 
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Default By the book GPS location

I can't recall the exact wording but my GPS manufacturer's instruction was something very close to 'An uninterrupted view of the horizon and free from EMI.'

Near as I could see the only spot I could get that was on the tail, so that's where mine went.

The hangar sharks circled and the frenzy began regarding flow disruption and flutter etc. I studied similar shapes, talked to a few engineers, and went ahead keeping the 'E' in experimental.

I am still here after approaching Vne several times splitting the true airspeed difference when testing was at altitude and can report a very happy location there on top of my tail.

So even when one comports to the letter of the manufacturer's instruction there is still room to cite safety margins. Tradeoffs right?
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  #8  
Old 06-04-2021, 03:07 PM
azonic75 azonic75 is offline
 
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Default Another thing

One another little issue is that while failure of an IFR GPS in VFR conditions is a "nothing" you will lose your ADSB out WAAS position which can be a nuisance if you happen to be sitting on the ground at a big airport. Safety issue...no, it would be a PIA though
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  #9  
Old 06-04-2021, 05:28 PM
Handclutch Handclutch is offline
 
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Vans puts the GPS antenna under the cowling on the 12. Several hundred flying and no problems reported on the forum that I can recall.

Jack
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  #10  
Old 06-10-2021, 10:48 PM
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RV7A Flyer RV7A Flyer is offline
 
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OK, I'll play...

The GNS430W Installation Manual says exactly nothing about mounting the antenna under the cowling. Lots of "should" statements about avoiding masking by the airframe, distance from comm antennae, etc. But not a word that would cause an under-cowl mounting position to "violate" one of the *guidelines* (not requirements).
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