May 1, 2011
By Barry Schiff
As an 18-year-old flight instructor in 1956, I tried to be clever and original in the way I taught. There were times, though, when I was too clever.
A case in point is when I covered the static ports of a Cessna 180 with Scotch tape. The intent was to see if my instrument student would discover during the preflight inspection that I had sabotaged the airplane. Inspecting the static ports, I had found, was one of the most overlooked preflight items.
Somehow during all of this, I became distracted and managed to forget that I had taped the static ports. Murphy’s Law came into play and this was to be one of those times when my student would not be as thorough as he should have been.
The airspeed indicator operated normally during the takeoff roll. After liftoff and during the subsequent climb, however, neither the vertical speed indicator nor the altimeter budged. I quickly cursed myself because this situation was of my own making. I nevertheless feigned innocence to see how my student would react.
“So what’s the problem?” I yelled above the roar of the engine. This was when few light airplanes had intercoms, and cabin speakers were used instead of headphones for listening to the radio. Little wonder that so many pilots of that era experienced hearing loss.
My student knew immediately what caused the problem, although he did not know why. “There’s something wrong with the static system,” he yelled back. “I’m going to open the alternate static source.” As soon as he twisted open the petcock, the altimeter and VSI sprang to life and indicated normally.
In a sadistic sense, it would have been more fun had the airplane not been equipped with an alternate source of static pressure. We could have played, “Guess Your Altitude,” a game that one of my early instructors taught me. After a short period of maneuvering in the practice area with the altimeter covered, he would ask me to guess our altitude. This is more challenging than you might imagine. He would then ask me to descend to and level off at 2,000 feet agl (also without benefit of an altimeter). Anyone who thinks this is easy has never tried it.
Many pilots think that an airplane has two static sources for purposes of redundancy. Not so. If that were the case, an airplane would more likely have two pitot tubes because these are more susceptible to blockage than are static sources.
Assume, for example, that I had taped only the right static source. During a left slip, the relative wind would come from the left of the nose, and this would tend to blow air into and slightly pressurize the left static source. The result would be an increase in static pressure that would cause the altimeter and VSI to indicate a descent, even though the airplane might actually be maintaining altitude or even climbing somewhat.
Conversely, a right slip would cause the left static pressure to decrease. This would cause the altimeter and VSI to indicate a climb even though the airplane might actually be descending.
Should you ever experience a situation during turbulence when the altimeter and VSI become extremely sensitive and fluctuate wildly between high and low indications, this most likely is caused by the blockage of one static source. By gently yawing the airplane one way and then the other, you can easily determine which static source is blocked.
This explains why most airplanes are equipped with dual static sources. They balance or equalize the pressure variations that occur on both sides of the fuselage during the slipping and skidding caused by turbulence or the improper use of rudder. (One exception to this rule is the Cessna 172, which has a static source on only one side of the fuselage.)
Consider that a partially or totally blocked static system also affects the airspeed indicator. Depending on circumstances, indicated airspeed could be more, less, or exactly as it should be. This is when it would pay handsome dividends to know what combination of attitude and power is needed to safely make an approach to landing without having to rely on an airspeed indicator.
As fascinating and instructive as it can be to demonstrate the consequences of flying with a partially or totally disabled static system, consider that blocking one or both static sources for this purpose is likely a violation of FAA regulations, because this would result in flying an airplane that is not airworthy.
Although most airplanes used for IFR flight are equipped with an alternate static source, some of these —and many VFR-only airplanes—are not. One way to resolve the problems created by blocked static ports in such aircraft is to carefully break the glass of the vertical speed indicator. This would allow static air pressure from within the cockpit to enter the static system. The altimeter would then operate more or less normally. If the VSI is not damaged in the process of breaking the glass, consider that it will indicate backwards because the “flow” of static air pressure into the instrument would be reversed. A positive VSI indication would indicate a descent, and vice versa.
Believe it or not, a local instructor recently told me about a pilot who had asked if static pressure could be restored to the instruments by similarly breaking the primary flight display (PFD) in a “glass” cockpit. No joke.
Barry Schiff has been an active flight instructor for 55 years. Visit the author’s website.
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