IAS Station Zero

The airspeed indicator will sometimes roll over and die, but you dont have to, too


Every pilot in the known universe has heard the term flying by the seat of your pants. And until I interviewed the late Captain Elrey B. Jeppesen a few years ago, I thought I understood its meaning.

But Captain Jepp set me straight. During the early years of airmail flying, of which he was a part, instrument flight wasnt even on the drawing board. Jepp explained that the early mail pilots, tooling along cross continent in open cockpit biplanes, were on their own when they encountered weather. Maps by Rand McNally, no IFR rules or clearances, no radios and, critically, no gyros.

Yet even through horrific winter weather, pilots managed to get the mail delivered. Thus it was thought at the time that keeping an airplane upright in the clouds without instruments of any kind was some kind of innate talent that some pilots had and some didnt. At worst, seat-of-the-pants flying was a skill that could be taught and learned. No science supported this because there was no such science.

An appalling number of fatal crashes proved the theory wrong and, by the early 1930s, gyroscopic instruments – which Captain Jepp said were actually resisted and mistrusted by many old-timers – became the accepted means of flying without visual references.

Today, pilots are so dependent on gyros and other basic flight instruments that when one rolls over or becomes otherwise unavailable, theyre disoriented at best, fatally lost at worst.

Seat-of-the-pants is not merely discredited, it has been left for dead. Thats unfortunate, for it still has a place in everyday flying.

A Runway Excursion
Consider a recent accident in which a Twin Cessna ran off the end of runway on takeoff, struck a berm and came to rest in a ravine, severely damaged and with serious injuries to the occupants. Witnesses said the twins engines sounded normal on run-up and takeoff and the airplane appeared to accelerate as expected. One witness watching the takeoff said he couldnt understand why the Cessna didnt rotate, despite having traveled two-thirds the length of a 4,500-foot runway at what appeared to be normal speed.

The reason could have something to do with what the investigators found after the airplane was extracted from the ravine: Both pitot covers were still in place, their remove-before-flight banners wrapped tightly around the pitot masts. Although the preliminary accident report is unclear on this point, it appears that the pilot saw 80 mph on the airspeed indicator and was waiting for 105 mph to rotate. More puzzling is how – or even if – the pilot saw any airspeed indication at all, given that both covers were securely fastened and free of holes.

Assuming the engines were operating normally, the simple moral of this story appears to be this: Those remove-before-flight flags really mean what they say.

But theres a less obvious question to ask: Is pilot training now so scattershot that we really need a working airspeed indicator to takeoff and fly an airplane? It sometimes seems so.

But it doesnt have to be that way. Even during the course of routine flying – en route cruise or the Saturday morning joy pop – you can conduct simple training drills to develop and sharpen your feel for an airplane such that in some circumstances, you can trust the seat of pants rather than instrument indications that may not be available.

Airspeed Fixation
The death-grip reliance on airspeed indication must certainly evolve from stall awareness and avoidance training. Pilots are generally taught that the airplane stalls at a certain airspeed, theyre counseled to avoid that speed and thus begins the obsession with the airspeed indicator. Lost is the notion that its a certain angle of attack that causes the stall; airspeed is but a secondary means of measuring the increasing angle of attack.

Instructors can and do teach the sight picture method of recognizing and avoiding stalls. The view over the glareshield should look … like so, power off … and theres the stall. But on takeoff and approach, that lesson recedes as the pilot reverts to setting attitude via airspeed.

Breaking the habit is easy. Cover the airspeed indicator with an instrument blocker and go fly. Do some slow flight, do a stall series, some steep turns and – more important – pattern work and landings. (Instructor optional.)

Sans airspeed indication, you can sense takeoff and approach attitude just as the old timers did: By looking out the window. You dont have wind in the wires to judge airspeed but every airplane has its unique sound level. For this exercise, you dont care about the difference between 68 and 74 knots. Youre after too fast, too slow or about right.

Like a blind person whose hearing becomes more acute, youll focus more sharply on control feel and sight picture over the nose for certain configurations. On landings, the initial tendency with no ASI is to fly the approach with power, flat and too fast.

So what? Bleed off the airspeed in the float and try again, whittling the speed back by sound and feel until you reach a comfort zone. That may be five knots faster than your normal approach or right on the money, based on control feedback and feel.

Takeoffs without airspeed indication are even easier to do by feel. Trite as it sounds, an airplane – any airplane – announces its readiness to fly, sometimes subtly, sometimes not. A Cessna 150 with the trim even close to takeoff setting fairly hops off the runway when it reaches flying speed.

Other airplanes – heavier types, such as a Bonanza or Cessna 210 – need a little more coaxing, but not much. Slight backpressure during the takeoff run will eventually nudge the nosewheel off the runway, indicating that sufficient flying speed is but seconds away.

At some point, the speed will become so great that the airplane will have to be forcefully held on the runway. Not to put too delicate a point on it, but if you cant recognize that, maybe you should take up bowling instead.

And When It Lies
The most insidious aspect of airspeed indicator fixation is not when it quits entirely but when it indicates erroneously in IMC, with no visual cues to cross check. This stuff is considered so basic that its given a few pro forma questions on the IFR written and promptly forgotten, in some cases by experienced pilots who ought to know better.

Its reasonable to assume that some number of GA accidents have occurred because of ice-blocked or malfunctioning pitot/airspeed systems. Without CVR/FDR data, the accidents are written off as unknowns or ascribed to some other cause.

But there have been at least two airline accidents caused by erroneous airspeed indications and the picture painted by the CVRs is none too pretty. One occurred on December 1, 1974, when a Northwest 727 broke up and crashed within minutes of departing Kennedy Airport on a ferry flight with only the three-man crew aboard.

Within days of recovering the flight recorders, the investigators pin-pointed the cause. The 727 launched into icing conditions but the F/E neglected to activate pitot heat, causing both tubes to freeze over. As any student pilot knows, this will cause the airspeed indicator to act like an altimeter, since air pressure trapped in the pitot lines is at a fixed value and expands against declining ambient pressure as the aircraft climbs. Since its nothing but a pressure gauge, the ASI sees this as increasing airspeed.

When the crew of the Northwest 727 saw both indicators showing increasing airspeed, they were obviously baffled but did what comes naturally: Pitched up and continued the climb, figuring the airspeed indications would fall into line. They didnt. Seeing even more rapid acceleration in airspeed, they continued the pitch up until the airplane stalled and spun, killing all three crew.

More recently, a chartered 757 was lost off the coast of the Dominican Republic under remarkably similar circumstances. But the blockage was suspected to have been caused by mud or insect debris. The wreckage sank in 7,200 feet of water, so this couldnt be confirmed.

Although the 757s sophistication is light years beyond the 727, both airplanes were brought down by errors in simplest and most fundamental systems: the tubes and gauges that show airspeed. Despite having an attitude indicator the size of a dinner plate, the 727 crew evidently never cross referenced it against ASI readings that made no sense. If they did, they didnt believe what they were seeing.

The 757 crew had a working standby airspeed indicator and evidently knew there was something wrong with the primary system immediately after takeoff. Yet over the course of some five minutes of confusion leading to the crash, they could not between them sort out the baffling ASI indications and lost the airplane as a result.

Recognizing It
Sophisticated airline simulators can duplicate blocked pitot indications, although its unclear how often such drills are actually carried out. The 757 charter crew had not been recently trained – or perhaps trained at all – to deal with this anomaly. What chance, then, do you have?

Plenty, actually. Things happen more slowly in a Skylane climbing 500 feet per minute at 110 knots than in a 757 at 220 knots and 2,000 fpm.

The first indication of blocked pitot with trapped pressure will be nothing. In level flight, everything will look peacefully normal. Upon pitching up for a climb, however, the airspeed will increase at a relatively steady rate. Upon seeing that, cross check against the attitude indicator and the VSI. If both confirm pitch up, the pitot is blocked. (The VSI should remain unaffected by pitot blockage.)

The situation can get more sporting when you initiate a descent and see the airspeed indicator show decreasing speed. More nose over, less speed, more nose over, and so on. Response should be the same. Cross check the attitude instruments and VSI and adjust accordingly. That may initially include a level off until you can sort things out, checking the pitot heat in the process.

Unfortunately, theres no ready way to simulate this in a light airplane. Well, there is one way. But you may not have the stomach for it.

On a day with high bases and low tops, find a light icing layer and fly for awhile with the pitot heat turned off. Pitot probes and masts are efficient ice collectors and if theres ice there, they will ice over. Guaranteed.

Whether they trap pressure or not may be determined by random chance. The tube may be just as likely to vent the pressure and drop to zero just before it freezes solid.

If this suggestion strikes you as a suicidal risk, forget it.

Dont even consider it. Write us an angry letter and tell us how appalled you are. But do know that some pilots and instructors have done and are doing it as a training exercise. One wonders if that charter crew would be alive today if they had been exposed to such edge-of-the-envelope experimentation.

Well never know. But we do know that despite having expensive training and a multi-million dollar jet with redundant systems out the wazoo, they were ultimately no better prepared than the Twin Cessna driver who happened to overlook a couple of pitot covers.

-by Paul Bertorelli

Paul Bertorelli, a CFII and ATP, is editor of The Aviation Consumer.


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