Stall testing after a modification and stuff that can go wrong

sixpacker

Pre-takeoff checklist
Joined
Dec 20, 2014
Messages
163
Display Name

Display name:
Sixpacker
So enough time has passed that I can now tell this story. :lol: No, I didn't do anything illegal or something like that. Maybe just a little dumb. I will be the first to admit that I've done plenty of dumb things in my life. The plan is as I get older that I will do fewer and fewer stupid things and learn from my mistakes as I go... So far so good.

Anyway, enough rambling. This particular episode occurred while I owned a 206. I have to admit this is a plane I loved and I miss it. It was a little slow and expensive to operate fuelwise. The engine maintenance was high (I see a thread in the maintenance section on expensive cylinder work right now... Alex I feel your pain been there done that) but besides that the rest of the airframe was inexpensive to maintain. I could practically go anywhere with that plane and almost carry anything. Please excuse me I'm rambling again.

Back to the story. I decided to get the tip tank modification. Flint Aero tip tanks. The tips add three feet to the wingspan but for that you get an almost 200 lbs of gross weight increase. So even if you're not going to use them for holding extra fuel you get to legally load an extra body on your plane.

Have a look at this article though then come right back ;)

http://www.aviationlawmonitor.com/2010/03/articles/general-aviation/the-trouble-with-tip-tanks/

I read about this before ordering the kit so while I'm sure Flint Aero is not thrilled by this story at least in my case it did not affect my decision to get it anyway. I don't know if the installation procedure changed as a result on that particular accident. Either way the installation procedure calls for a fairly detailed process for adjusting the stall warning after installation. More on that later.

I ordered the kit and arranged to have it installed by a local shop. Before I handed it all over I read the installation instructions intensively and was well aware of this stall warning adjustment procedure called for in the STC instructions.

The local shop worked on it for two - three weeks. The kit also involves installing pumps, switches to operate the pumps from inside the cockpit and also gauges (which surprise surprise don't work worth a damn). Lights need to be moved and rewired, the tips painted, etc. So a good solid 2-3 weeks worth of work.

When the work was completed I got the call and excitedly headed on over. I inspected the work and it all looked good. "So did you adjust the stall warning vane?" I asked. The mechanic looked at me blankly. So I suggested we have a look at the stall warning adjustment procedure because I'm not sure I wanted to hear the stall warning going off on every take off and landing. So we looked through it...and discussed how it was going to be done. He said he would need to get someone to fly the plane since he is not a pilot. No problem, I said, I will of course fly the plane. Then he said he will need to have someone fly with me and record the results. No problem again, I said, YOU will fly with me. No, he said, he doesn't like test flights. And, he said, he absolutely detests the 206 because there is no door on the right side and he wouldn't be able to get out easily. :eek: At this point I got a little nervous. Why would be have to get out? What does it mean when the A&P doesn't want to fly in the plane he just worked on? I may have said that if he is not prepared to fly with me then I am going to lose confidence in his workmanship and request him to restore the plane to it's original state. I don't recall exactly but I was almost certainly diplomatic. There was some further discussion and then he agreed he would fly with me.

So now that this was decided, we can move on to the actual process itself. It was nothing complicated, it really was just concerned with making sure that the stall warning sounds a few knots above the stall speed for all configurations and at max gross weight. I forget how much about the stall speed but it was something like 5 knots I think. For the no flap and full flap configurations and at most forward CG and most aft CG I was to hold level flight then slowly bleed off speed at about 1 knot per second and note when the stall warning went off and then when the wing actually stalled. So a pretty typical stall test but a lot slower with the power reduction than usual. We computed our weight and balance. We filled up as much fuel as we could carry (120 gallons). The two of us up front at this point brought us well in front of the CG front limit. We were both large individuals. He was fat and I'm big boned and muscular ;) and least that is my story and I'm going to stick with it. The fact is though we had a lot of weight up front. Luckily we were not at our gross weight limit. So what to put in the back.... Hmmm.... How about sandbags? Yes, let's put sandbags in the back. So we put in enough 25lb sandbags to bring us up to the new gross weight limit. We check the W&B again and we are close to the front CG limit and at max gross weight. Okay good to go! The A&P rotates the stall warning vane by the suggested amount. If we are lucky, the tests will confirm this new position otherwise we will have to adjust and then repeat.

So up we go. Hmmm, I had to use a lot more back pressure than usual to get off the runway. Maybe my A&P friend is a little heavier than he admitted? I notice he is sweating. He is heavy though and heavy people do sweat more than the usual. Maybe I shouldn't read too much into that? All in all it is just a fleeting concern and I don't think any more of it as we progress to the test area we had decided on.

We set up for the first test. This one is with no flaps. I set up level flight then start reducing power very slowly. Pull back to maintain altitude. Hey, this is kind of fun I was thinking I've never done it this slowly before. Keep decreasing. The stall warning starts to sound. Keep dropping the power slowly. Approaching 5 knots. We stall. But this is not like the usual gentle stall. The plane pitches down abruptly and kind of caught me by surprise. I rationalize it though knowing we are near the front limit. The plane is severely nose heavy and as the wing stops flying I surmise the nose drops rapidly down enough that the rear stabilizer stalls rapidly as well and the front drops even quicker. No worries though I recover the usual way. I have to do this three times. Each one gets more abrupt. Now onto full flaps. We set up for the first one. Reduce power slowly. As expected the stall warning comes on at a lower airspeed. Keep decreased. Then we stall. And boy did we stall. The nose pitch downed rapidly and I mean rapidly and before I could react we were staring directly at the ground. We must have been briefly pointing 80 degrees down from horizontal. I do not recall how much height we lost before recovered but it was considerable. The A&P screamed some string of obscenities that I will not repeat here and insisted that the test was over. I did not disagree. Any more rotational inertia as the nose dropped and I could imagine the plane ending up a$$ backwards! And upside down. Unusual attitude anyone? So what does happen with sandbags moving around the interior as you do these kinds of motions anyway?;)

We flew back to the airport and I demonstrated quite possibly one of the worst landings I've ever done it was shall we say a firm landing. Firm but safe. Don't you hate it when you have a terrible landing and your pilot buddy says "at least it was safe"? You know they're agreeing that it was a terrible landing.

Yes, as suspected the sandbags had shifted. So some things I learned from that day: Flight testing with moveable weights is not the best idea. Doing stalls with a CG in front of and outside the envelope was quite educational. Stalling at a low altitude in this condition would have been fatal. Stall with a too rearward CG would be even more exciting...
 
Last edited:
I've heard of post-maintenance stall fight tests getting very exciting to the point where the sidewalls and headliner came loose in the said ~25,000 pound pound jet losing 10k feet altitude in the process.
 
FWIW, I don't like to fly in the right seat of a 206 either. And that rear door is a PITA to open with the flaps down -- which is of course exactly how you will be configured after an emergency landing.

The issue with unsecured sandbags is a good one. I have to admit I've flown an aircraft with those in the back, too, though not for a post-mod test flight. Thanks for the heads up.
 
...I may have said that if he is not prepared to fly with me then I am going to lose confidence in his workmanship and request him to restore the plane to it's original state...


If you had lost confidence in his workmanship why would you request him to work on your airplane again?

Seriously though, there are A&P Mechanics who are not pilots and have no interest in flying. Personally I don't get that, why you would want to be an A&P if you weren't a pilot or at least had some interest in flying, but I have known quite a few and some were not only disinterested, they were downright scared of it. :dunno:

Entertaining story though, I enjoyed it.
 
What about the CG shift as fuel was being burned, did you guys allow for that? Wouldn't CG move outside of the forward limit if you were on the limit to start with?
 
Seriously though, there are A&P Mechanics who are not pilots and have no interest in flying. Personally I don't get that, why you would want to be an A&P if you weren't a pilot or at least had some interest in flying, but I have known quite a few and some were not only disinterested, they were downright scared of it. :dunno:

Entertaining story though, I enjoyed it.

I'm one of those A&P/IA's who never got a pilot's certificate but I love to fly.

Never was afraid to fly but there have been some pilots I won't fly with.
 
If you had lost confidence in his workmanship why would you request him to work on your airplane again?

Seriously though, there are A&P Mechanics who are not pilots and have no interest in flying. Personally I don't get that, why you would want to be an A&P if you weren't a pilot or at least had some interest in flying, but I have known quite a few and some were not only disinterested, they were downright scared of it. :dunno:

Entertaining story though, I enjoyed it.

At that time I didn't really know. Interestingly, though, the following year I did my annual with someone else and I was not so pleased with what I found. First of all, this first guy had decided to power the lights in the new gauges by "stealing" the power from the pedestal lights. I only fly maybe 10% of my time at night and I had noticed that the pedestal lights weren't working but it wasn't a big deal and I didn't link it to the tip tanks work. I kept on meaning to get it fixed and never did until the next annual then I found out why. Not good but got that fixed. The second issue is that when the A&P mechanic (the heavy guy who didn't like flying with me :lol:) drilled the holes for the gauges he just left metal shavings on the other side which we then found during the annual. Also, not good. So in retrospect, not a great job. Got it all fixed up though.
 
What about the CG shift as fuel was being burned, did you guys allow for that? Wouldn't CG move outside of the forward limit if you were on the limit to start with?

In the 206, CG shifts to the rear as you burn fuel so we were good. A detail I omitted though is that you get a 200 lb gross weight increase but the landing weight does not get increased. I really needed to burn 200 lbs before returning to land... which I didn't do :redface:
 
In the 206, CG shifts to the rear as you burn fuel so we were good. A detail I omitted though is that you get a 200 lb gross weight increase but the landing weight does not get increased. I really needed to burn 200 lbs before returning to land... which I didn't do :redface:

Thanks, interesting. I wonder what the stats are for GA aircraft in that regard, aft vs. forward CG shift in flight.
 
In the 206, CG shifts to the rear as you burn fuel so we were good. A detail I omitted though is that you get a 200 lb gross weight increase but the landing weight does not get increased. I really needed to burn 200 lbs before returning to land... which I didn't do :redface:

I was curious about this as well. There is no mention of it in the POH that I have ever been able to find and I haven't experienced it in practice either. Visually it appears that the fuel tanks and nearly centered with CG point. If I plot the fuel load moment is decreases slightly as the fuel load is reduced indicating a slight forward CG shift. Also, I was wondering how it is possible to be out of forward CG with sand bags anywhere behind the pilots seats? I know you don't have the 206 anymore, but here are the charts I'm referencing:
 

Attachments

  • Screen Shot 2014-12-31 at 6.18.03 AM.png
    Screen Shot 2014-12-31 at 6.18.03 AM.png
    214.6 KB · Views: 8
  • Screen Shot 2014-12-31 at 6.18.38 AM.png
    Screen Shot 2014-12-31 at 6.18.38 AM.png
    117.5 KB · Views: 9
  • Screen Shot 2014-12-31 at 6.19.29 AM.png
    Screen Shot 2014-12-31 at 6.19.29 AM.png
    185.7 KB · Views: 13
How does CG move aft as fuel burns? My Cessna's have had essentially the same arms as the 206 and their CGs move forward as fuel is removed. Same with Cubs. The change forward is greater when lightly loaded than when heavy to the aft limit but even with max aft CG the CG moves forward as fuel is burned off. At least that's been the case in my airplanes.

Re:reference in a POH? Do a few W&Bs with the only variable being your fuel load. How's it differ between full and less than full tanks? You're expected to know.
 
Last edited:
Just as a point of comparison, the Bonanza F33A (an aircraft I used to fly a lot) has a fuel burn that shifts CG to the rear.

This is a serious consideration and one of the nice aspects of the 206, no CG problems.

Here it is clearly referenced in the POH:
 

Attachments

  • Screen Shot 2014-12-31 at 7.24.45 AM.png
    Screen Shot 2014-12-31 at 7.24.45 AM.png
    135.3 KB · Views: 7
  • Screen Shot 2014-12-31 at 7.28.43 AM.png
    Screen Shot 2014-12-31 at 7.28.43 AM.png
    118.1 KB · Views: 5
I was curious about this as well. There is no mention of it in the POH that I have ever been able to find and I haven't experienced it in practice either. Visually it appears that the fuel tanks and nearly centered with CG point. If I plot the fuel load moment is decreases slightly as the fuel load is reduced indicating a slight forward CG shift. Also, I was wondering how it is possible to be out of forward CG with sand bags anywhere behind the pilots seats? I know you don't have the 206 anymore, but here are the charts I'm referencing:

It's been a while so I don't have all the exact details at my fingertips. What I do remember though is that with the tip tanks full and two big guys up front the CG goes forward of the limit. The CG for the tip tanks for some reason is slightly forward of the main fuel tanks so that hurts as well when you fill them up. Perhaps we didn't have enough sand bags and then allowed them to shift forward? I don't know but what was obvious was that the plane was VERY nose heavy. Try some stalls sometime with the CG very far forward it is quite eye opening. :eek:. No don't :lol:
 
I was curious about this as well. There is no mention of it in the POH that I have ever been able to find and I haven't experienced it in practice either. Visually it appears that the fuel tanks and nearly centered with CG point. If I plot the fuel load moment is decreases slightly as the fuel load is reduced indicating a slight forward CG shift. Also, I was wondering how it is possible to be out of forward CG with sand bags anywhere behind the pilots seats? I know you don't have the 206 anymore, but here are the charts I'm referencing:

On that W&B chart, extend the top line to 3800 lbs and then extrapolate that left angled line to intercept. That is what the new chart looks like. And as you can see at the new gross weight you have a narrower CG range and easy to get too far forward.
 
Plot your intended gross weight CG Arm using your takeoff fuel load. Re-plot the CG arm with less fuel. Does the Arm go decrease or increase? It decreases. CG moves forward. That's the only way it can work since the 206 datum is the firewall and the wing tanks are at +47". The variable with 206s is the individual aircraft CG Arm but I'm sure it won't be aft of 47". My own zero fuel CG is 40" and fuel is added at 48", just for reference.

In my 180 it's possible to get my CG out of limits forward when I'm on wheel skis, empty, and light on fuel. Looking at the airplane with the heavy skis in the retracted position it's easy to imagine. A wheel plane is much more difficult to get out of CG forward. All pilots should be aware of how loading affects their own CGs. Personally I use my Aviation Weight & Balance app all the time. I have my own airplane empty weight data saved for tires and skis. How I load the plane is an important factor in how it performs. It matters.
 
Last edited:
Could it possibly be that the wing never stalled? Instead, the tail stalled first, lost lift, and this caused the unexpectedly high pitch rate to nearly 90 degrees down?

OK, waiting for the barbs to fly....

Stan
 
I've heard of post-maintenance stall fight tests getting very exciting to the point where the sidewalls and headliner came loose in the said ~25,000 pound pound jet losing 10k feet altitude in the process.

I would guess the seats need to be cleaned also. :eek:
 
Back
Top