Safety At Low Speeds

Plane & Pilot - June 1982


The Lift Reserve Indicator can help you fly more confidently during critical approach and departure situations


By Keith Connes

If I were to ask you how fast your plane can fly, you could probably tell me within a knot or two. But if I were to ask you how slow your plane can fly, you might have a harder time answering. That information is rather important, because there is a point at which any airplane transitions from a flying machine to an aluminum brick, and we don't want to reach that point unwittingly.

The trouble is, that point can not be read with any degree of certainty from the airspeed indicator. As you know, a given airplane's stall speed varies with the weight it's carrying, flap setting, and angle of bank, among other things. Moreover, ASIs tend to be unreliable at the lower end of the scale because of the pitot's increased angle of incidence to the relative wind.

Knowing this, we tend to fly our approaches too fast, adding a few knots for mother and the kids. On top of that, the pilot's operating handbook has its own margin built in for the aircraft manufacturer's defense attorney as well.

You might think of this as a margin of safety if you are flying a small plane onto a big runway; but the day may come when you have to set your plane down at Goochtown Airstrip (or maybe even a potato field for unexpected reasons), and you could find yourself creating an airplane-shaped opening on the fence at the far end. Even at Bigtown International, it's nice to be able to decide for yourself the place at which rubber meets concrete. Moving back to takeoff time, you will also want to know just when to rotate and exactly where Vx (best angle of climb).

For some time the Safe Flight Instrument Corp. has been marketing an angle of attack indicator, which is a more sophisticated version of their basic stall warning system. This is probably installed on the plane you fly.

More recently a system called the [Lift Reserve Indicator]...has been developed...According to its inventor, Morgan Huntington, the [LRI] does not measure angle of attack, but instead measures the potential wing lift that's available prior to the onset of mushing sink, which is the precursor to the stall.

Unlike the Safe Flight system, the[LRI] does not require electricity...

Huntington feels that what the pilot really needs to know for lift management at slow speeds is what he calls POWL - the potential of wing lift. This is the amount of lift remaining that the pilot can utilize by pulling back on the yoke prior to the onset of mushing sink. The [LRI] measures and displays the potential wing lift under all flight conditions.

...The instrument uses as its point of reference the differential pressure that exists when the airplane is flared at the instant of touchdown. This is what Huntington calls POWL-zero - the point at which no surplus kinetic energy is available. When the instrument has been properly calibrated, the needle will be at the bottom of the white arc at POWL-zero. It is important to understand that the needle will be in this position just prior to mushing sink regardless of the plane's airspeed, weight, center of gravity, flap setting or angle of bank.

At that point, if the pilot were to pull back on the yoke, the needle would drop into the red sector, indicating that he could no longer reverse the sink with up-elevator pressure. At a specific location near the bottom of the red sector, the plane would stall. The exact needle indication will vary from one aircraft model to another, but the important point is, if the needle goes into the red sector you are approaching a stall.

The [LRI] has been given extensive scrutiny by both the AOPA Air Safety Foundation and Cessna's engineering department. John Sheehan, recent director of the flight instructor department of the AOPA Air Safety Foundation has been flying in the foundation's Turbo Centurion installed with a [LRI] for about a year and a half. Sheehan has more flying time with the device than anyone else.

P&P: John, what do you think of the Huntington Airspeed Director [LRI]?

Sheehan: The bottom line is that is allows you to operate the airplane to its maximum level of performance without doing any guess work, in much the same way you can with an angle of attack system.

P&P: Morgan Huntington feels that there is a considerable difference between his indicator and an angle of attack indicator.

Sheehan: From an ex-Navy carrier pilot's standpoint, it is an angle of attack device. I understand what Morgan is saying. It does give you a little more than angle of attack in the dynamic range, although it may not be readily apparent.

P&P: Have you flown the [LRI] in comparison with say, Safe Flight's angle of attack system?

Sheehan: Yes, but not in the same airplane. We have a B33 Debonair that has the Safe Flight indicator, and I prefer the [LRI] because it's a little more qualitative in the information it provides. It gives you a more rapid indication of what the airplane is doing and provides a broader speed range.

The big advantage is that it has just one moving part, and no electrical supply required...I like the[LRI]'s presentation a little better too.

P&P: Exactly how do you use the [LRI]?

Sheehan: On takeoff I know that at the division line between the red and white sector on the indicator, which is at an alarmingly low airspeed, I can drag the airplane off the ground at any weight. If I hold it at that line, the airplane will climb at best angle of climb.

P&P: And this will be lower than book speed?

Sheehan: Oh, yes. It's a bit scary. Then you can accelerate to best rate of climb speed until midway in to the green, and if you use book value, 97 knots, you're cheating yourself. Granted, it's a small amount, but it could make a difference if you were in a tight situation.

In maneuvering flight, it may be of very little value, but once you get into the pattern again, if you make all your pattern turns between the yellow and white sectors, you'll never get into any problems. One of the advantages of the device is that it gives you almost instantaneous response - there's very little lag time in it.

P&P: Will your approach speed also be lower than book, using the [LRI] in that way?

Sheehan: Absolutely. The book speed for the 210 is 73-75 knots, I think (author's note: it's 70-80 for a normal approach and 74 for a short field approach), and flying the [LRI] you'd be about 64-68, depending on your weight. That may seem like a lot, but it means a lot in terms of float - and you still have full control over the airplane, with more than enough elevator authority to get a flare.

P&P: Are you using the [LRI] in some sort of training program?

Sheehan: We took some low-time pilots who had never flown a 210 and had them fly the [LRI] in the pattern, disregarding the airspeed indicator. Believe it or not, they made good landings first time around, and in one case, there was a good-sized crosswind.

Then we covered up the [LRI] and told them to fly with the airspeed indicator using book value. They kind of messed it up because they were going way too fast and floated, and they had to fool around with the flare. Things like that happened.

Acting as instructor pilot, I've flown with maybe 20 people who were using the [LRI] for the first time, and I've never seen anybody really chase it. I've never seen them prang the airplane unless they over-rotated the airplane into the flare, which some people do. The big thing was, they didn't float, and they had much more precise glideslope control than they would have had otherwise.

P&P: Do you now use the [LRI] as your primary low-speed instrument?

Sheehan: Absolutely. I don't use the airspeed indicator anymore for takeoffs and landings.


P&P: ( Dave Ellis Cessna's supervisor of advanced design and systems research) Dave, can you tell me what you have done with the LRI?

Ellis: I've worked in the area of angle of attack instrumentation and stall avoidance for some time, and I've flown most of the instruments that are available. I was reasonably impressed with the LRI, so I asked Morgan to give us one that we could play around with here. We put the probe unit in the wind tunnel at Wichita State, and then put it on our prototype 152 along with the standard vane-type angle of attack system, so we could compare the readings.

P&P: Huntington claims that there is no weight dependence, but you feel there is.

Ellis: Yes, but I don't consider it a serious shortcoming of the instrument.

P&P: Now let me turn the question around. How does pressure differential compare to angle of attack as a reliable indicator of stall?

Ellis: You can use pressure differential with a probe like Morgan is using, or you can make an arrangement of pitot tubes that measures differential pressure, or you can put pressure sensors on the leading edge. All of theses have been done.

If you took a pressure differential and somehow divide it by the pressure you get out of a pitot tube, you would have a true angle of attack indicator. By not dividing it by dynamic pressure, you get a variation in the instrument reading due to that dynamic pressure.

P&P: And that's what happens with Morgan's indicator?

Ellis: That's right. You can prove it by just driving the airplane down the runway in a level attitude so the angle of attack doesn't change. The needle will come of the peg and work its way up, and all its reading is dynamic pressure.

Or, if you fly the airplane at a constant speed and change the angle of attack, then Morgan's differential pressure is just giving you angle of attack change; but if in the process, you change speed, it will give you the speed change as well, just like an airspeed indicator.

P&P: What about approaches? Does the [LRI] give you the correct margin above Vso for your approach when you are flying say in the middle of the white arc?

Ellis: Yes, it does. I like to think of it in terms of the energy that you have available to flare the airplane, and it tells you that quite reliably.

P&P: The aircraft manufacturers' book speeds for the approach seem to have a pad in them. Do you agree?

Ellis: I would agree with that. The book numbers are based on a maximum gross situation which you seldom land the plane at, and they're padded a bit. The numbers in our book are going to work even in some turbulence, and they're certainly going to be conservative at light weight. What Morgan's instrument does is give you a way, without doing a lot of computing, of flying the airplane at the right speed for the weight you're at.

P&P: I gather from all of this that you like the instrument.

Ellis: I'm pretty positive about it right now. I don't see anything about it, if it is properly installed and calibrated, that could lead you astray. On the contrary, I think it would be a positive safety asset in the airplane, especially for the heavier airplanes - the 200 series and twins. It would help you obtain maximum landing performance safely under all conditions.

One other thing: I think it could be a very valuable device for training. For the first time, you've got something that will really show the student angle of attack. You're not asking him to use his imagination. The [LRI] has all the instincts of an angle of attack indicator, so you can show him what happens in a high stall. I think anybody learning to fly would be safer in an airplane equipped with one of these instruments.


Fen Taylor is an engineer at Mooney. His company has an [LRI], but has not evaluated it yet. (1982) However, Fen has flown the [LRI] and was willing to express his thoughts:

P&P: Fen, what do you think of the [LRI]?

Taylor: Well, this is my personal opinion...I think it's relatively easy to use, and is based on sound engineering.

P&P: How do you feel the [LRI] compares to an angle of attack indicator?

Taylor: It's a little different from an angle of attack indicator, because what it really does is indicate the proximity to the depletion of kinetic energy. I flew with it, and it seems to do everything Huntington says it will. It took me a few times to get used to it, but after three or four landings I found I was able to land pretty precisely with it. It's a viable way of measuring the difference between the airplane's present speed and altitude, and where it starts a sink rate that's too fast to be acceptable.

P&P: Did you also find it useful in determining best angle or rate of climb?

Taylor: Yes. After two or three times, I found that I could pick out the best angle or rate, and hang in there very close. I think that it has to be fine tuned for each model of airplane, but if that's done, I think it's a good indicator.


I (the author of this article - Keith Connes) tried stalls under various conditions of flap, power setting and bank angle, and can attest that the stalls occurred at a consistent needle position, deep in the red sector. I also shot some landings and believe that the [LRI] is definitely useful in helping the pilot establish and maintain the correct attitudes for takeoff, climb, pattern maneuvers, approach and landings. That's a lot to say, but the [LRI] does a lot.

The display, incidentally, is mounted atop the glare shield where it is in the pilot's direct line of vision while he is looking at the runway - yet another advantage of the LRI.

...In sum, I think the [LRI] is a unique and useful instrument that has the potential of saving some of the lives that are lost in stall-spin accidents. It should be of particular value on a twin, where departure and approach speeds are more critical, and where single-engine situations require exacting speed control.