Flying without an ASI
By: Doc Green
The airspeed indicator (ASI) is probably the instrument in a plane that gets looked at more frequently and with greater interest than any other. This is especially true during approach and landing because the airspeed that is held on final approach is critically important. Too little, and you may stall or develop a high sink rate.
But what would happen if the airspeed indicator should fail? Can you still fly the plane with confidence? Is this a reason to panic? Pull the Big Red Handle?
Airspeed indicators are very reliable but they sometimes do fail to function properly. And it doesn't take much to render one inoperative. A tiny drop of water or the remains of a bug that get inside the instrument, or a stopped-up pitot tube ... simple little things can remove the ASI from your inventory of usable instruments.
Whether a failed ASI represents a crisis or a reason to panic depends almost totally on whether you have prepared in advance for this very situation. If you have, it's no problem, an unevent. If you are not prepared, it may be more exciting but there's a way out of even this unhappy situation.Fly by Pitch Attitude
At a given power setting, the location of the nose of the plane relative to the horizon serves as an excellent airspeed indicator. The reason for this is that if you place the nose at a certain point, the plane will come to an airspeed that corresponds to this position. The higher the position of the nose, the lower the airspeed will be.
On final approach, how do we control airspeed? By raising or lowering the nose of the plane. We do not control airspeed with the throttle. The throttle setting determines our rate of descent at the airspeed we have chosen to fly. If we give the plane a lot of power, it may fly level or even climb. Less power means we will descend. All this happens while holding the same airspeed.
So what we need to do is to learn the pitch attitude the plane will have at various airspeeds. And in fact, we do not have to worry about "all" the airspeeds. Our primary interest is in the region just above the stall. Where will the nose be relative to the horizon as the plane approaches the stall? Once we know this, all we have to do to avoid a stall is simply keep the nose of the plane below that certain point. And that's it!There are Stalls, and There are Stalls.
In training and in doing stalls for practice, we typically pull the nose of the plane way up to get a sharp, easily recognizable "break" as the stall occurs. The nose then pitches downward, and a wing may drop, which only adds to the fun. However, this is not the type of stall we have to guard against while flying a final approach with perhaps no airspeed indicator. We "know" that in that case, we're not going to be yanking the nose up really high.
An inadvertent stall from a glide is one that creeps up on you. We get into the situation by holding the nose a bit too high, which gives us a low airspeed. And if we continue to hold the nose just slightly too high, the airspeed will continue to drop off. But the plane is not likely to break as it does in a training stall.
What is likely to happen is this. First, the sink rate of the plane will increase as the plane begins to "mush." We may not notice this because of our altitude, unless we have an accurate vertical speed indicator and then only if we happen to look at it. The sink rate gradually increases as the plane slows even more with the nose being still "too high."
At this point, some buffeting may begin, the controls will be sloppy, and the nose of the plane may start to twitch back and forth from one side to the other. But, with aggressive rudder inputs, we can still hold it reasonably straight. Being distracted by the sudden "bumpy ride," we may pull back on the stick to counteract the tendency of the nose to drop. This only makes matters worse, and we may not even be thinking "stall."
As the sink rate accelerates, it will soon become impossible to keep the nose from turning. Finally, the plane will drop off sharply to one side or the other, totally out of control. What got us into this "creeping stall?" Holding the nose too high and not recognizing the high sink rate. (You may wish to read about an "Oscillating Stall" in an article on this site, Stalls 201.)
And how can we avoid this unhappy sequence? Don't hold the nose "too high." This brings up the obvious question, "How can I know if the nose is too high or not?" Fortunately, it's easy. Read on.Finding the Critical Pitch Attitude
Take your plane up to a safe altitude and then, after riding around a bit, reduce the engine power down to an RPM you may use on a typical final approach. Further, while maintaining this RPM, place the nose of the plane to where it needs to be to give the airspeed you hold on a typical final. Then, study that nose position. Memorize it. Learn it. What does it feel like sitting in the seat? Are you slightly forward? Slightly back? Straight up? What does the stick feel like at this speed and power setting? This is your reference point.
You may wish to do this several times just to get the hang of it. See if you can reproduce the airspeed without looking at the airspeed indicator.
Try a slightly different power setting and see what difference a bit of power makes in the pitch attitude required to hold the airspeed on your value. Once again, we're concentrating on flying an approach, so a lot of power is not in order.
Now the fun begins. Using the airspeed above as the starting point, once you're established in a stable descent at whatever RPM you choose, raise the nose just a bit, just enough to lower the airspeed by perhaps 5 mph from the value used above. Again, look where the nose is. It will be higher than it was before. Study this nose position, ... and so forth.
Then, take it down to an even lower airspeed. This will have the nose even higher. Study it; learn it. And then repeat.
With a good-working airspeed indicator, you will know the approximate stall speed of your plane so you will have a pretty good idea of where you are relative to the stall in these tests. There's no reason to take the airspeed all the way down to the stall. Just learn that nose position when it's, say, 5 mph above stall, and then keep the nose lower than that, and things should go nicely.
The nose position for minimum airspeed will vary with RPM, but the lowest, critical nose position will occur at idle. If you remember this one position and then keep the nose lower than this, you will be fine as well.Flying the Approach, Generally Speaking.
Two things go together to form the heart of a good approach: (1) Few corrections are required; and (2) the required corrections are small. The ideal, perfect approach would have the plane lined up perfectly from the time the turn to final is made, and with the airspeed and power setting "on the money" to carry the plane to the touchdown point without "any" corrections being required.
What we're aiming for is a final approach where there is little to do but sit and watch. If you find yourself busy on final, perhaps you're doing too much. Work and practice to find that magical combination of altitude, power, and airspeed, and then let life be simple!
Yes, I know that the wind can make things more eventful, and no two approaches are the same. However, we can strive for the ideal even though we may never reach it. Don't be a throttle jockey, and don't be always yanking the nose up and down. That only makes it more difficult. Work toward getting there with as few small corrections as you possibly can. But, if you get into a tight spot, don't hesitate to add power quickly or do whatever it takes to salvage the situation.We're Flying, and the ASI goes South!
First of all, there's no reason to panic. We know how to deal with the situation. Just keep the nose of the plane lower than that critical level, and all will be well.
But, what if it's been a while since we've done the "learn the pitch attitude" tests? Or, we meant to do them, but never got around to it? Well, no time like the present! Climb up to a safe altitude if you aren't already there, and then throttle back to the RPM you plan to use for the upcoming final approach. Keeping the nose well down at first, establish a stable descent at a constant airspeed. You will be able to tell if the airspeed is increasing or decreasing by the sound and feel of the plane. If you have a vertical speed indicator (VSI), aim for a descent rate of about 500 feet per minute.
Convince yourself that you can control the plane in a descent without an ASI. Then, raise the nose "just a tiny bit" and hold this attitude for a short time. See what happens. Give the plane time to stabilize. Be on the alert for buffeting or other indications of a stall, and watch the VSI if you have one. If you get an indication of a stall or a high sink rate, lower the nose, apply power, and recover. Then try it again.
The objective is to find the nose position that will keep you flying at the RPM of your choice. Then, on the final approach, the real deal, keep the nose of the plane a tad lower than this.
If you're an experienced pilot but one who has perhaps leaned too heavily on the ASI, you can move through these tests rather quickly. Just keep raising the nose until you get the very first indication of a stall. But do this very gradually so as to keep the airspeed more or less stabilized for the particular pitch attitude you may have at a given time. You don't want to be zooming or diving.
Once you identify the nose position that gives the slightest indication of an approaching stall, that's the one you want to remember. I assure you, it takes less time to do this test than it does to write about it!
Conventional wisdom says, "If you lose your airspeed indicator, come in hot on final." Well, up to a point, this is OK. However, just remember that once you get to the runway, you've gotta touch down and stop. If you're red hot, this phase of the whole affair will be made much more complicated by the excess speed. And in the worst case, you could over-run the runway and wind up going off the end.
The best possible thing a pilot can do is to prepare for the loss of the ASI ahead of time by practicing all these things, with an instructor if that's what's called for. Once you've done it a time or two, it becomes no big deal. And that, after all, is the objective.Airspeed Indicators in ULs are often Inaccurate.
This is just my observation, based on a very limited number of planes. The inaccuracy is probably due in large part to the difficulty of getting a good static pressure. The ASI basically compares the pitot pressure to the static pressure and derives the airspeed from this information. If the static pressure is not truly static, the ASI indication will be in error.
The possibility for inaccuracy means that you cannot depend upon the indicated airspeed in absolute terms. Think about what this means when you go to test fly a newly-built plane for the very first time. The situation is little better than having no airspeed indicator at all! That is, until you get the plane in the air and have a chance to check out stall speeds and so forth, you really don't know how to interpret the indication of the ASI.
To illustrate this point, when a friend of mine (old dude, referred to in the next section) first flew his pretty new plane, I asked about the airspeeds he got with the new engine and prop setup he was so proud of. Without batting an eye, he reported, "It took off at 20, cruised at 30, and landed at 20 again."
The problem was with the static air source. So he simply disconnected the static line so that his source of static pressure lay inside the cockpit area. Indicated airspeeds then went from being way too low to being way too high, by 15 mph at an actual 65 mph. And they still are. He likes it that way. Makes him think he's going somewhere fast! Yep, he goes really fast but doesn't get there any sooner.
When you climb into an unfamiliar plane, even one of a familiar type, you should be wary of the airspeed indicator until you get some confirmation about its accuracy. This could come from another pilot who has flown the plane and who can vouch for the ASI.
Sometimes, of course, an ASI will seem to work "just like the last time I flew" but the indication will be way off from what it was then. This rarely happens, in fact, but the possibility is always there, and you won't know it until you are in the air, or you are in the process of falling out of the air.
One little strip I fly into frequently is tricky at best. Tall trees block one end and preclude the possibility of a last-minute go-around. Further, there is a stand of low trees not more than 100 yards from the touchdown point so that you can't fly the plane in at a very shallow angle.
You have to hold an absolute minimum airspeed on final or you will go into the trees at the end of the strip. (It's happened!) That is, if your plane stalls at 32 mph, fly the approach at 37. There is no room in this situation for an undetected airspeed indicator problem to show its ugly face.
So, what I do routinely on the way to that strip (which is near a restaurant down on the lake, i.e., food and pretty faces), is to throttle back, slow down, and see what the airspeed indicator says when I feel the beginnings of a stall. If it says 32, ... I feel better. That's what it was last time."Are You just Making this Up?"
No, not this time. I've lost the ASI more times than I can remember in GA planes, and once while flying an ultralight. But it's no big deal. In the GA planes, I know the pitch attitude where I'm OK. So I fly by the pitch angle and don't worry about the ASI.
Here's the story of my ultralight adventure with the ASI, which happened in May of 2003 when I had not more than 2 hours of solo time in ultralights of any type. (But I had close to 15 hours, dual and just riding.) My instructor, an old dude, had just finished a new plane and was eager to take it to a fly-in some 50 miles away. At the same time, he wanted me to fly his old one over in the hopes that he would find a buyer for it. Well, what the heck? Away we went.
We arrive there, uneventfully, land, talk airplanes and tell lies. We admire the bull's eye drawn around the spot where the Challenger, after touching down a bit fast, hit the side of a cinderblock building near the end of the strip. We eat, tell more lies, and then eat some more, for the better part of the day.
Finally, it becomes time to depart. I then observe that we've got a 15 mph crosswind to go along with the uphill takeoff that's going to be required to get out of this bumpy little backwoods strip that runs parallel to a line of trees. OK, I've got almost 3 hours in this plane now. Shouldn't be any problem. Yeah, right!
Instructor takes off first, with me watching. His wings bounce around a bit as he lifts off right at the top of the little rise in the runway, but ... it could've been worse. I do my run up and then watch the wind sock until it shows a bit less wind, then it's full power and hang on!
I'm fully focused on staying out of the trees, keeping the plane on the upwind side of the strip. My speed builds rapidly as I approach the top of the hill. I'm about to be airborne, but I just glance at the airspeed indicator just to be sure I have plenty of airspeed before I lift off into unknown territory. Airspeed indicator reads ZERO.
Now, in less than one-half a second, I reason this way. This plane is new to me, and I'm an inexperienced ultralight pilot. And my ASI does seem to have malfunctioned. Should I abort, or go on? Yes, that's the question alright. I think I can fly this thing without an ASI. After all, I've got 50 miles between here and home to learn the position of the nose. And if I abort, the old dude will have to turn around and land to see what's my problem. Also, if I abort, I'll have to make this takeoff run again, and next time, I might not even get this far before going into the trees, wrecking this plane, and then having to buy it for scrap metal and cleaning rags. This is a no-brainer. I'm flying!
So off into the air I go. No problem. A quick crab into the wind prevents any sideways drift toward the trees. I just keep the nose a bit lower than usual to maintain plenty of airspeed, sit back, and enjoy the ride.
On the way back home, I keep my little secret to myself about the ASI being on the blink. No use to worry the old dude. As we get close to the Big County Airport, he suggests we stop in there and see what's going on at the little shindig they're having. And we do. In landing on that full mile of asphalt, I just keep the nose down, fly down next to the runway and let it settle onto the pavement in its own good time. Meanwhile, I'm watching the position of the nose to see what it does as I get slow, down next to the runway, that is. Comes up quite a bit, in fact, and with the plane still flying. I make note of that.
On the ground, we eat, tell lies, then eat some more, but I say nothing about the ASI problem. But I'm thinking about it, because the approach at our home strip is dicey to say the least for a GA pilot, and I haven't landed there more than a half-dozen times, ever.
Now, if I tell the old dude about his ASI not working, it will just worry him. Or he will want to swap and have me fly his new plane with a different engine, and it's all new and shiny, ... and I don't want to wreck that one either. So I keep quiet. Then, in due course, it's time to go. Oooh Kay.
On the way over, old dude decides to circle the house of a flying buddy, to show off and let him know that we can fly in the wind. While he's doing that, I'm practicing stable descents, and feeling for the stall, and studying the nose position. I find that at 3,500 RPM, if I hold the nose "right here" or lower, I should be just fine. Finally, it's time for the big show. Flying a tricky approach into a strong headwind with no ASI.
I plan a long final, maybe a mile. I start the approach about 400 feet AGL, carrying a bit of power because of the wind. I ease the nose up to "just below" the critical point, and hold it there. Seems OK. VSI is not reliable, but it's not doing anything really bad in any case. Plane seems stable at this attitude. Controls are firm. Hold everything right where it is.
As I get close to the point where I have to drop it in over the trees, I realize I'm a bit high. I back the power off to idle and lower the nose just slightly. Plane feels solid, even with the wind bouncing me around quite a bit. Continuing, I clear the trees and then nose down fairly sharply. This causes my airspeed to increase, but I regard this as life insurance. I cross the end of the strip a little fast, but the plane settles in for an acceptable landing. It's good to be home!
(A photo of this strip is in the article "Getting Lined Up on Final" in this same section on this website. It's the very last photo in the article.)
After putting the planes up, I mention to the old dude, "Hey Capt'n, I hate to tell you this, but your airspeed indicator is on the blink."
He looks at me and with no hesitation says, "Aw, that little tube's done come off again."
"What little tube?"
"The one that runs from the pitot and connects up behind the instrument panel. It comes off every now and then. Just stick it back on and it'll be OK. Don't take but a second."