Need advice and opinions on this... slow airspeed

[NT5224]

Hi folks!

 

Im just putting a query out there to the brains trust. Many of you are much more experienced than I and  can bring a second  (and additional) opinions to my own conclusions regarding an incident over the weekend.

 

Here's the story. Sorry if its long but I'm including all potentially relevant details to give context

 

So my bird went in for its annual this weekend. Its a great aircraft but I mentioned  to my LAME that the last time the wings were detached for work the  control lines hadn't been accurately tensioned, so the ailerons did not appear level when the stick was centered. And to maintain wings level in the air  I was flying with the stick very slightly to the left of centre. I showed him this minor issue and asked if he could adjust during the inspection. Everything else was fine. 

 

I picked up the aircraft  over the weekend. Doing my walk around I saw the ailerons appeared my level with the stick centred. But I  didn't see my LAME, just took off and flew home. In the air  I immediately noticed it wasn't flying quite straight. It felt like I was flying with rudder in just to keep straight.  It made be wonder whether he had overcorrected the tensioning, or I was just feeling the difference because I was used to flying it as it had been before. Not sure.  It was nothing dangerous, but we just seemed to be crabbing a little.  Retrospectively I also recall my airspeed was rather low on the flight home.  

 

But things got really interesting when I came into land. Landing at my place in the wet season is kinda like landing on an aircraft carrier.  On the approach you are at 300ft then you are suddenly at tree top height as you fly across the edge of the escarpment (the strip is on an escarpment).  Flying my approach  everything seemed quite normal but checking my airspeed I saw I was at 45 knots coming onto final and as I slowed for landing this fell to 35 knots!  Normally I cross the threshold at about 50 knots and land  touch down at 45 knots, so 35-40 knots  seemed much too low. According to the operating handbook my Rebel will fly 34 knots with full flap, but I don't want to test that at 10ft over the trees, so  I put on power and went around.

 

My first thought was that maybe there was a headwind or something. so on my second approach I watched airspeed both on my panel and on the GPS (ground speed). Now,  if the  issue was a  headwind ( and there was no evidence for a headwind on my windsock), surely my airspeed would read true?  But both sets of data read consistently and again I was over the trees at 35 knots indicated.   Again this was too slow and I was spooked and went around for a third time.  I was actually worried and wondering whether i should head back to an alternate  (less challenging), airfield to land at, so to figure out the problem. But then something occurred to me. On my approaches indicated at 35 knots, the landscape didn't seem to be moving any more slowly than usual. If I didn't have the instruments and flew looking outside the cockpit  the 'picture' on approach looked quite normal. So I wondered if my gauge and GPS could be inaccurate (although it seems strange that both could be reading wrong  by the same margin). So on my third  approach I just flew like I always do, and didn't really look at the airspeed.  I landed fine.  I actually ballooned slightly and so it was a long landing because I subconsciously  landed fast (worried about the low indicated airspeed).

 

So ladies and gents, what's going on?  I have thought this through over and over.  The aircraft incorrectly configured, crabbing and flying slow,  the mysterious  'gulley winds' for which our district is famous, or somehow both my pitot steam gauge and GPS  both reading incorrectly. I struggle to believe my aircraft was actually flying at 35 knots, its never flown that slow before and it didn't feel sluggish or on the point of stall. And on my final landing I actually slightly ballooned which was unlikely to occur under 40 knots.  But  to my knowledge the LAME didn't so anything to the airspeed indicator.  Very strange indeed. 

 

Any ideas or suggestions? Obviously I will discuss with my LAME

 

Cheers

 

 Alan 

 

 

                 

 

 

 

 

    

 

 

 

 

Edited December 13, 2021 by NT5224

[IBob]

I would be looking at this as two entirely separate questions (albeit quite possibly connected):
1. Either your airspeed is indicating accurately, or it isn't. That can be checked at various speeds and at a safe height, allowing for some delay in the GPS etc.

2. Either your aircraft is correctly rigged, or it isn't. For my aircraft there are various measurements and also a metal template for setting up the flaperons.

 

It would also be extremely useful to know what if any parts of the control system the LAME touched.

 

It occurs to me that if he just tweaked down an aileron to match the other one, then you too now have flaperons! It seems very unlikely, but I had a car which nobody could get to run straight down the road, because they just kept tweaking one side or the other rather than centering both sides. So I guess anything is possible......

 

Most interested to know what you find...........)

[facthunter]

Check a static line hasn't been left disconnected. That doesn't address why the rudder is needed when  flying straight.  Its a fixed sheet metal tab on the rudder if I recall correctly and not easy to get wings level zero rudder.  It WILL vary with power setting . That's normal. Has a tab been bent?  Contact the LAME in any case.. Nev

[Ryanm]

Did your Airspeed/GS drop suddenly? While reading your post, Wind-Shear kept popping up in my mind. My thoughts are that an ASI error (sustained) would've become apparent earlier in the flight? You state the ASI and GPS readout were both indicating relative to each-other at all times?

 

It could be possible that the re-rigging of the Ailerons has induced a slight yaw effect. Maybe that's why they were rigged that way initially?

 

Interested to hear what ultimately caused your issues.

 

[Thruster88]

When flying a new type or just to check the airspeed indicator in my machines I stall and note the indicated airspeed. Indicated stall speed never changes for a given configuration and weight for a training type stall. Stall speed would not change more than a few knots even if the ailerons were both slightly dropped.  

 

If the aircraft now stalls at a lower than standard indicated airspeed there is most likely a leak in the pitot line. With a helper to watch the ASI blow gently into the pitot tube and block with your tongue at say 80knots, it should hold there.

Edited December 13, 2021 by Thruster88

[skippydiesel]

1 hour ago, Ryanm said:

Did your Airspeed/GS drop suddenly? While reading your post, Wind-Shear kept popping up in my mind. My thoughts are that an ASI error (sustained) would've become apparent earlier in the flight? You state the ASI and GPS readout were both indicating relative to each-other at all times?

 

It could be possible that the re-rigging of the Ailerons has induced a slight yaw effect. Maybe that's why they were rigged that way initially?

 

Interested to hear what ultimately caused your issues.

 

This is a likely investigative direction. If you dont have longitudinal trim on rudder, then a similar effect can be achieved by trimming with aileron. I have seen a cheap but effective trim (for heavy passenger) on one aileron tube/rod using a bungee cord attached to tube with zip ties, around a simple pully and  up through a sailing cleat, that allowed for "holding" in whatever tension was required.

[440032]

For the airspeed anomaly, I think I'd subscribe to Nev's theory - static line left disconnected perhaps.

[Thruster88]

4 minutes ago, 440032 said:

For the airspeed anomaly, I think I'd subscribe to Nev's theory - static line left disconnected perhaps.

For aircraft that have an alternate static source (usually inside the cabin) the airspeed indication is usually only a few knots different. 

[Bruce Tuncks]

The first flight after a service is a test flight, and you have done this well.

Personally, I suspect the ASI and you can check this out with a water manometer...  we used to do this on a glider annual.

Except for the static line, and you have got good advice to check this.

 

 

 

 

 

 

 

 

 

/

[kgwilson]

I'd check the ASI with a water manometer first for accuracy & make sure it holds (no leaks) but nothing can explain to me why the ASI and GPS were identical with little or no wind but everything felt wrong. You will get accurate results with the manometer at higher speeds. With low speeds the water travel is less so it is more difficult to accurately determine the correct speed.

 

Check everything and when conditions are favourable fly lots of circuits both using instruments and not using instruments and video them inclusive of the entire panel.

[Student Pilot]

Maybe the ailerons were rigged down a touch to try and allow for reflex once flying? If they were rigged lower that would make for slower flight and mebe slower stall speed. Rigging an aircraft is not a black and white thing there are nuances and very individual to each particular aircraft. Can take quite a bit of experimentation to get right. An aircraft (especially light/ultralight) can be flown easily out of trim just by a bit of different pressure on the rudder. Always good to test fly a few circuits before departing for your home field.   
How did the controls feel? Firm, like approaches before?

Headwind not going to make any difference to your indicated airspeed, only groundspeed.

 

 

[Yenn]

I do not understand what you re saying that it felt as if you were flying with rudder in to fly straight. A glance at the ball should be all that is needed. Did you have to fly with rudder to fly straight?

When the ailerons are equally placed the stick could be off its centre position, but that doesn't really matter unless it is way off. Are the ailerons even? You can usually check to see if they both align with the wing tip or the root end of the wing.

The airspeed problem does seem to indicate a leak somewhere. I wouldn't advise blowing into the pitot, unless you want to risk damaging it. A better way to check is to get a plastic tube that snugly fits the pitot head. Attach it to the pitot, block the other end to prevent an air leak, then gradually roll it up to increase the air pressure. This should show a gradually increasing air speed and if it increases, but then subsides you have a leak. If it doesn't increase you have a big leak.

Obviously you flew on attitude to get home and that proves the old saying that attitude is everything. If you feel happy enough to fly again, take it up to a safe height and check how it feels at several speeds as you approach the stall, but I suggest you check for a pitot leak first.

[NT5224]

50 minutes ago, Yenn said:

I do not understand what you re saying that it felt as if you were flying with rudder in to fly straight. A glance at the ball should be all that is needed. Did you have to fly with rudder to fly straight?

When the ailerons are equally placed the stick could be off its centre position, but that doesn't really matter unless it is way off. Are the ailerons even? You can usually check to see if they both align with the wing tip or the root end of the wing.

The airspeed problem does seem to indicate a leak somewhere. I wouldn't advise blowing into the pitot, unless you want to risk damaging it. A better way to check is to get a plastic tube that snugly fits the pitot head. Attach it to the pitot, block the other end to prevent an air leak, then gradually roll it up to increase the air pressure. This should show a gradually increasing air speed and if it increases, but then subsides you have a leak. If it doesn't increase you have a big leak.

Obviously you flew on attitude to get home and that proves the old saying that attitude is everything. If you feel happy enough to fly again, take it up to a safe height and check how it feels at several speeds as you approach the stall, but I suggest you check for a pitot leak first.

Hi Yenn

 

Yes,  the ball was off and I was applying  a noticeable left rudder to keep straight. However, prior to takeoff the issue of the uneven ailerons seems to have been remedied. At least they looked level. My LAME has since confirmed he adjusted them. I have no idea if my sensation of flying crooked and the unusual airspeed readings are related but one would assume they were. My aircraft flew perfectly before the annual inspection and ailerion adjustment, but slightly crabbed afterwards. However my LAME confirmed neither the Airspeed indicator or the GPS has been touched during the inspection.

 

On my LAMES suggestion (and on yours too)  I will try to clear the Pitot tube outward from the cockpit. Then I'll go up for a test fly,  get up high, try flying slow  see what indicated speed I get to  just before stall.  

 

This would be no big deal,  but for the GPS reading the same as the airspeed indicator. Baffling!

 

Alan 

 

     

 

   

 

 

 

 

[Student Pilot]

Maybe it was reading wrong before? Nil wind the airspeed and GPS should be close.

[Yenn]

You said it went in for the annual. It appears that the airspeed indications were different after the annual. If there was any problem with the ASI or its plumbing the LAME should have found that during the annual. I assume it is a Murphy Rebel which is I think too heavy for RAAus so you are stuck with LAME maintenance, unless you can do your own legally. I just wonder how good your LAME is, have you given him all the info you have given us?

[facthunter]

Rebel with 100 912 Rotax is not too heavy for RAA .  Done lots of hours in one. Nev

[APenNameAndThatA]

What did the person who did the rigging say? 

[Old Koreelah]

On 14/12/2021 at 1:02 PM, Yenn said:

Obviously you flew on attitude to get home and that proves the old saying that attitude is everything. If you feel happy enough to fly again, take it up to a safe height and check how it feels at several speeds as you approach the stall…

Good idea, Yenn. I know very experienced pilots who never cover their pitot, presumably because they don’t need the ASI. 

 

While stuck in my valley during lockdown I’ve been brushing up on basic skills like flying by attitude in case I need them when we finally get to fly long trips. Lots of practise flying near the stall might improve my chances if the noise stops. 

This morning I did a bit of flying around at V1.4 with flaps, doing tight turns and simulated EFATO turns onto the cross strip 1,000’ above the circuit.