Stall Warning

[Charles Schefer]

Any advice, particularly from twin owners would be appreciated....

Ever since I've had my PA-30 I've questioned the accuracy of the stall horn. It seems to come on VERY early. For example, on Monday, landing in smooth air / light xwind with 1/2 flaps and 90 mph approach speed the horn seemed to want to chirp almost constantly. Not being one to ignore a warning, I lowered the nose and increased throttle. No more constant chirping but a fast landing resulting in a long float in ground effect as one would expect. There were two of us up front and no ballast in the back so that did not help.

I've usually found my best landings are right at 80mph over the numbers with lots of nose up trim. I've gone back to using 1/2 flaps for landing vs full as smooth landings are more consistent. I've had plenty of approaches at 80mph with NO chirping of the stall warning, other than perhaps the occasional "blip". Conversely I've had the stall horn chirp in the pattern at speeds as high as 120 mph, particularly in turbulence which may explain it.

One thought might be - is the airspeed indicator accurate? Without going into great detail I believe it is. The airspeed indicator was overhauled not long ago and plane performs as I would expect for the indicated speeds. I've also cross checked it against the GPS (average of four cardinal headings etc..) and I believe the airspeed indicator is accurate.

Any thoughts on this topic and particularly what other twin owners experience would be appreciated.

One other related note (though not the main point / question of my post).... there is little that I don't like about my Comanche but one thing is the way the stall warning is a series of chirps or beeps instead of a continuous tone. It every other plane I've ever flown, a stall warning is a continuous warning horn. in fact when I stall warning horn intermittently "chirps" that to me has always been the indication that you are right on the edge of the stall (or a bit above the buffet). On the Cirrus I can usually get a few chirps from the horn in the landing flare just as the mains touch down. In the Comanche there is only the warning as a series of chirps... so there is no sound of the on-set of a stall... the sound is the same at the edge and full-on. I don't really like that but I guess that's the way they designed it.

- Charles

[N3322G]

Mmmm.

Suggest taking it to 5,000 and checking the stall speeds in various configurations against the airspeed gauge.

Another Twin owner had posted a few years back about having a hard time landing smoothly. Accurate airspeed was a problem and so was rigging.

[Charles Schefer]

Thanks Pat, that's a good idea.

- Charles

[N3322G]

FYI. Unless I'm fighting a stiff crosswind with partial or no-flap config, I land full flap so touch down speed is lower and yes, trim is usually full-up. a bit challenging for the occasional go-around but even with my relatively puny upper body strength it hasn't been a problem. My Twin lands well at 90 mph over the fence. I personally like the tiny stall chirps a second prior to wheel touchdown. It tells me I'm as slow as I can get for touchdown for lowest wear and tear.

Also suggest electric contact cleaner on the stall warning/lift indicator.

[Charles Schefer]

Thanks Pat. I was always using full flap except in Xwind but more recently I went back to 1/2 flap feeling I got a more consistently smooth landing. The other factor is I find the plane stays very "light on her feet" after touch down with full flap. I think I may experiment and go back to full flap on final. I note that you fly 90 over the fence? Since VMC is 90 (well in your PA-39 it's probably 80 mph) 90 makes sense. I always fly final at 90 but then slow to 80 over the numbers or I feel like I float too much.

- Charles

[steen]

Charles,
The stall warning has no connection to the airspeed indicator. It is actuated by a vane on the left leading edge of the wing and is only concerned with angle of attack, not airspeed.
Concern with VMC ends with the "commit to land" point of the approach, after which you can close the throttles and land it like a real airplane with no regard to engine loss.....you have committed to land.
I agree that stall testing will show if the vane is incorrectly indicating stall angle of attack as compared to airspeed, but this
is only valid at idle power as the thrust of partial power corrupts the info. The stall warning should come on before the actual
stall buffeting by a few mph ias.
steen

[Charles Schefer]

Thanks Steen. I'm well aware of the stall vane - I test it on every preflight with power on and lifting the vane with my finger to ensure the horn is working. I do that during my walk around checking lights and pitot heat. I wasn't suggesting the warning was connected to the pitot tube at all. My points were twofold:

1) Main Point: It seems odd to me that on some approaches in landing configuration with partial (final approach) power and an IAS of 90mph that I sometimes (but not all the time) get constant chirping from the stall horn. Assuming my airspeed indicator is accurate, and I currently believe it is, then the stall warning seems premature. Adding to this I sometimes get an occasional chirp of the horn at 120mph on downwind turning base. This latter aspect may be due to the of course higher stall speed in bank and general I've only had that when its bumpy and the chirps come with the the "bump" updraft of turbulence.

2) Minor point: I wish the stall warning was a constant tone not a series of beeps but that's just me.

I agree with your commit to land philosophy and idle power stall point comments. After all, to land the wing has to stop flying. I agree with you and Pat in that doing some stall tests at 5K (power on and power off stalls) would be a good idea and I'm going to do that.

Charles

[Charles Schefer]

Well I went up to 5000 MSL and did some stall tests and am quite happy with the results...

In a clean configuration, power back to idle, the stall warning started around 90 mph indicated. The buffet at the onset of the stall came right at the bottom of the green arc - 76 mph (and I mean literally right on the mark at the bottom of the green arc).

In the landing configuration, gear down and full flaps, the stall warning started at 73-74 mph and the buffet at stall on-set came right on the mark at the bottom of the white arc at 69mph, and again I mean right on the mark!

I was very pleased that the the stall buffet came right on the predicted marks of the airspeed indicator for the specified configurations, it wasn't even slightly off it was right on. The stall warning in the landing configuration with full flaps came at 73-74 which is just 4-5mph before the actual buffet which seems right. In the clean configuration it was a bit more eager coming about 14mph before the buffet. Perhaps this is due to the stall vane not being far (laterally along the wing) from the left windmilling prop.

So... I mentioned that I was getting the horn at 90 mph on landing but I also mentioned I'd gone back to using 1/2 flaps and the stall warning seems more eager coming on earlier in a cleaner config. So I went back to full flaps for landing. I made a squeaky smooth landing with full flaps, full nose up trim and NO stall horn beeping on final. Flew short final at 90 (ICS AFM actually recommends 95) and transitioned to 80 mph over the numbers sinking into the flare. Horn didn't beep once until my wheels were just touching down.

Thinking about this more... 1.3 Vso would be 89.7mph (1.3 x 69mph dirty stall). So 90 on short final seems right IF full flaps are used. My flying final at 90 with only 1/2 flaps is somewhere less than 1.3 Vso. So I'm now pretty happy with my stall warning. At some point soon I'm going to augment it with an AOA indicator - probably in an Aspen PFD but for now I'm happier having done some testing and feeling I've got it all figured out.

So lesson learned for me - I am going back to using full flap for landing, unless there is a good crosswind in which case I will use 1/2 and increase speed on final 5-10 mph.

While I was out practicing, I also did engine shut downs and flew around on just one. Restart, back up to temp and smooth.. shut the other side down. Flew beautifully on one engine at 5000 feet as has always been my TC experience. To be fair it was cool and the plane was not fully loaded so performance was good.

- Charles

PS - one follow up comment. I also did some slow flight in the landing configuration - full flaps, gear down and power to hold altitude right at the bottom of the white arc (69 mph). Flew beautifully. Stall horn was intermittent but mostly off but I had power (not idling props). So this again makes me think the prop speed does impact the stall vane even though it's a ways out laterally from the prop it may still be a bit influenced by the wash.

[SLIMDREDGER]

Charles, When and if you decide to install an Angle of Attack indicator, consider mounting above the glare shield so you can see it out of the corner of your eye when you are eyeballing the runway. I first installed a 3" one in the top row of gauges in the panel as it was the only size on the market when sold by the original developer in the early 1990s. Later on a 2" gauge was marketed and I changed to the smaller instrument. All the markings were in the top half of the gauge and I built a bracket that held the gauge with the top half projecting through the glare shield in the saddle between two top row instruments. (Frees a hole in the top row for something else, too ! ) I think having the gauge above the glare shield much better than having to look down below in the panel during the approach and landing.

Al Powers N88AP

I

[Charles Schefer]

Thanks Al, I think you make a very good point.

- Charles