Thursday, January 7, 2010

Heat on Pitot Tubes

There's an article about pitot tubes in the current (January 6, 2009) issue of Aviation Week and Space Technology. We start to learn about pitot tubes during Flight Lesson One. Learning about pitot tubes changed my academic career, at least a little: in pure Mathematics, we don't need instruments, and we certainly don't learn that instruments need to calibrated.

Like any instrument, a pitot tube is designed to work under specific conditions. AF447, which crashed into the Atlantic Ocean last year, had pitot tubes that were certified to handle temperatures as low as -40C, although it is not unusual for turbine airplanes to fly in colder air. For example, the King Airs I flew were limited to outside air temperatures above -53.9C; certification down to -40C is not enough. Now, some are calling for pitot tube certification down to -70C, because temperatures that low occur in the very high thunderstorms of the Intertropical Convergence Zone. Nobody seems to know what shapes ice crystals take at those temperatures, and, besides, it's hard to keep something warm when the ambient temperature is that cold.

The scandal is that this deficiency was first identified in the mid-1990s, but the airplanes still have inadequate pitot tubes.

I am reluctant to confess that I have never had a good attitude toward pitot heat. That overstates the case, because my attitude is consistent with whatever airplane I'm in. In something like an Archer, that is, a single without deicing equipment, I don't think about pitot heat much, because I am going to work like crazy to stay out of any icing conditions. (The few times I've had airplanes like this in the ice I turned the pitot heat on.)

The first airplane I flew that could handle ice was the Piper Seneca II. We had four of them, flying three scheduled night IFR routes in Idaho and Utah, and I was the check airman. We all knew that the pitot heat didn't work well, and expected it. You would be flying along at 25" manifold pressure (which was good for about 130 KIAS in level flight at our altitude), but the airspeed needle would hover around 60 or 70. We joked about it, but as check airman I made sure that everyone knew the target power settings for each maneuver. When the airspeed indicator failed you were pretty sure that you could put the gear down as long as you were flying level at the right power setting.

This kind of thinking might have helped the crew of Birgenair 301 in 1996. The airplane left Puerta Plata, Dominican Republic, at night. Their's was not a pitot problem -- the static ports were taped over -- but the pilots (both the human kind and the auto- kind) were confused by the false airspeed readings. They are the reason my Seneca pilots knew that if they were 8 degrees nose up and had 31.5" of manifold pressure at 2450 RPM then the indicated airspeed was about 120.

It's an early instrument lesson: PITCH + POWER = AIRSPEED. Keep this in mind and a pitot tube failure is easier to handle.

I never flew a King Air with the pitot heat off. Not once. We turned on the "hot five" after cleared for takeoff: both fuel vent heats, both pitot tube heats, and the stall warning heat. Look at the picture: you can see that ice protection is very important in King Airs. And, the King Air has fantastic instrument redundancy: it has at least two of everything, powered differently, with redundant power sources and redundant buses ("dual-powered buses"). Not to sound redundant, but it was easy to be comfortable. Or even complacent.

I have seen a lot of pilots (myself included) chasing airspeed indications. We all know that's bad, and we know how to avoid it: use the right pitch attitude. One place that happens a lot is with engine failures. Best glide speed! people say, and raise the nose until the airspeed says, say, 60. The problem is that because of instrument lag, when the airspeed indicator says 60 the airspeed is really more like 50. Too slow! they say, and lower the nose until the airspeed says 60. Only now the airspeed is more like 65. There might be a number of oscillations. The pilot is focused inside instead of outside, and is generally behind the airplane. It's easier to set up the right pitch attitude, find a place to land, and only then fine tune the airspeed.

I often cover the airspeed indicator when failing the engine on a student. That's two lessons: instruments fail, and PITCH + POWER = AIRSPEED. Both taught long before instrument training. Maybe they should be taught well after, too.

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At January 7, 2010 at 4:33 PM , Blogger Dave Starr said...

Some excellent thoughts here, and thanks for the pint out on the ASWT article, I'm going to enjoy reading it.

Yeasr ago when I worked for the USAF I had the privilege of spending some time with one of the earliest scientific advisors to the GPS program. He was hard over on the idea that GPS could go much further into the real of the cockpit, including replacing the "archaic" (his word) pitot static system. He couldn't believe, and I can't either, that with the instrumentation technologies available today we would still even be thinking of pitot/static systems.

Perhaps from a pilot's perspective (anything that can go wrong will go wrong), his thinking was a bit too impractical ... but he certainly was on the right track.

We shouldn't even be thinking of the problems of deicing pitot tubes at -70C. That would be like spending research dollars to study the NACA radial engine cowling's performance at Mach two.

There are so many other methods we could use, redundant methods, that could measure speed and lift/angle of attack(a primary reason we want to know airspeed in the first place) that I truly think the industry is barking up the wrong tree ... stuck in a Pandora's box from the 1920's.

Let's get rid of pitot tubes now, certainly on high performance aircraft ... they are the wrong tool for the job.

(by the way, the USAF lost a 1.2 billion dollar super-sophisticated B-2 on Guam less than 2 years ago due to the pilots' following faulty airspeed data caused by, what else, moisture in the pitot/static system.)

At January 8, 2010 at 8:25 AM , Blogger Dr.ATP said...

Thank you for your comments. I forgot all about the B-2 accident, which seems more complex because of its complex flight control system. As I understand it, the wet sensors provided BOTH false airspeed AND false angle of attack information. In such a case PITCH+POWER=AIRSPEED doesn't work.

Another disturbing part of this accident was that only some crews knew that the B-2 needed pitot heat on while operating in humid conditions. Where was the mechanism to pass this crucial information on to everyone?

As for whether we still need pitot tubes, I think we'll have to disagree, at least in part. While in most conditions, airspeed can be computed from other parameters (descent angle, power, AoA,...), these computations may not handle transients (ie, gusts) well. You still need to have an input from the dynamic air pressure.

What's really a mess is barometric altimetry. This is the root cause of cold temperature errors, among other things. But barometric altimetry is great for separation purposes; GPS altitude is not, unless you base separation on which satellites each aircraft is using. Altitudes derived from different satellite constellations might have different errors, which might lead to a loss of separation.

At January 12, 2010 at 12:43 AM , Anonymous Claus said...

May I ask you a general question, please?
I am looking for mathematical models/modelers of air traffic control. For example, how to adjust speeds or trajectories (vectors) such that certain constraints are satisfied.
There are some engineers writing on such things, but they generally make the things more complicated than they are.

Thank you in advance,

P.S. An example of an fine analysis of car-traffic is

On traffic flow at a T-junction: a sufficient condition for a left-turn lane. Mathematical and Computer Modelling 24, No. 7, 53-57
Mr. Mesterton-Gibbons.

I found your blog following traces of Mr. Horonjeff's book on airport planning, which has the same publisher than your own book.

At January 14, 2010 at 1:41 PM , Blogger Dr.ATP said...


Thanks for reading. I'm sorry to say that I have no experience in ATC modeling. As a rule, though, engineers write at the necessary level of complexity.

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