Erroneous Indications — Avoiding Confusion in the Cockpit

By Robert N. Rossier, EAA 472091

This piece originally ran in Robert’s Stick and Rudder column in the March 2020 issue of EAA Sport Aviation magazine.

The other day I had the opportunity to review a video about NASA’s X-31 vectored thrust experimental fighter program and the accident that caused the loss of one demonstrator aircraft in January 1995 (the pilot ejected safely). Like most aircraft accidents, it wasn’t a single factor that caused the crash but rather a combination of factors and conditions that led to the disaster. And it reminded me of how easy it is even for a highly experienced pilot to get caught in any number of traps.

One factor that played into the X-31 crash was the fact that the conditions the day of the accident included high moisture levels, including visible moisture (read that as clouds), which is not the norm for the desert region north of Edwards Air Force Base in California. The aircraft had recently been fitted with a new pitot probe designed to operate better at the high angles of attack common to the X-31, but the pitot heat had not been connected. During the flight, the pitot tube began to slowly ice up, causing erroneous readings that contributed to the impending disaster.

Communication was also an issue. A problem with the radio mic control in the X-31 created a lot of static, which meant the chase pilot could not effectively hear what was being said or join in the communication between the X-31 pilot and controllers on the ground. Had this not been the case, he could have verified the airspeed for the X-31 pilot and confirmed the erroneous readings.

Ultimately, on the flight back to Edwards, the pilot lost control of the X-31 and had to eject. The automated systems tried to regain control, but the aircraft slammed into the desert floor, leaving a trail of scattered wreckage. Of course, there is much more to the story, but if an airspeed indication problem can foul up a highly qualified NASA test pilot, imagine how such a situation might go for one of us mortal pilots.

The pitot-static system for a light general aviation aircraft is not particularly complicated. The pitot tube measures the dynamic pressure of the airflow, while a static port measures the ambient pressure of the surrounding air. The difference between the two pressure readings is displayed by an indicator calibrated in units of airspeed (knots or mph). Not a heck of a lot typically goes wrong with this system, except for blockages by ice or the meddling of insects that seem intrigued by small holes. We inspect the pitot tube and static ports before flight to make certain they are clear. If flying IFR, we check that the pitot heat is operational. But that is it.

Unfortunately, we don’t know if the airspeed indicator is working until we’re well into our takeoff roll — if we’re paying attention. The inattentive or distracted pilot might not recognize a problem with the airspeed indicator until airborne. If that pitot tube is fully or partially blocked, the pilot could suddenly find him or herself with confounding and confusing information being presented, which at low speed and altitude is not a good situation to be in.

That situation is just one reason why we need to include a check of airspeed indication early in our takeoff roll. If we detect a problem, an aborted takeoff may be in order. It’s much easier to deal with it on the ground than in the air, especially if we’re trying to clear obstacles or enter the clouds.  

So, what happens if our pitot-static system becomes blocked? As you may recall from training, it will act like an altimeter. It reads steady in level flight, shows a decrease as we descend, and increases as we climb. If the pitot tube becomes blocked with ice during flight, it might show a reasonable airspeed in level flight. But, again, a climb or descent will cause it to increase or decrease, respectively. This could be confusing, particularly for a pilot flying in IMC.

Should the blockage happen on the ground, we might see the airspeed slowly register as we climb out on takeoff, but likely it would show an alarmingly low airspeed, which might cause a pilot to push the nose down.

One way to help deal with the confusion that can come with faulty or missing airspeed readings is to know the numbers — the configurations and power settings that result in known airspeeds. Remembering the old saying — pitch plus power equals performance — if we know the pitch and power setting for any particular climb speed, such as cruise climb or VYSE, then all we need to do is set the pitch and power, and we’ll know what airspeed should be indicated. Likewise, we should know the power and pitch settings that give us our typical cruise and approach speeds. The same goes for descents. We should know the power setting and pitch attitude that gives us a standard descent at cruise speed or at approach speed.

The airspeed indicator isn’t the only instrument that can send us on a fool’s errand. If our static port becomes blocked, it will no longer indicate changes in altitude, which can put us in a seriously confusing and dangerous situation. If we recognize the problem, we can open the alternate static air port in the cockpit, which opens the system to cockpit air pressure. The altimeter may read a smidge higher than it should because the cockpit is typically at a slightly lower pressure than ambient, but it puts us back in business.

Another way to get a reading on our altitude is to apply full throttle and then read the manifold absolute pressure (MAP) gauge (if so equipped, and for nonturbocharged engines), which reads in inches of mercury. At full throttle, the MAP will be close to ambient pressure, and knowing that atmospheric pressure decreases about 1 inch per thousand feet, we can quickly calculate our approximate altitude. Since sea level absolute pressure is about 30 inches, we subtract the MAP indicated pressure from 30, and that’s our altitude in thousands of feet. If the MAP is 24 inches at full throttle, we’re roughly at 6,000 feet.

Flight instrument failures can also heap a generous supply of confusion on a pilot, especially at night or in instrument conditions. Typically, the attitude indicator is vacuum driven, so if the vacuum pump fails, the attitude gyro winds down, becoming lazy at first and then gives up the ghost entirely. But if we cross-check our instruments, we can readily determine when one isn’t behaving normally. For instance, a standard rate turn with constant airspeed tells us we’re in a level turn with a roughly 12-degree bank, and the power setting and airspeed indicator also confirm that attitude, even if our attitude indicator says otherwise.

The take-home message here is that we need to be proficient in dealing with erroneous indications, whether it’s the airspeed indicator, altimeter, or other instruments. It’s one thing to deal with these situations during training, but they catch us by surprise when they occur randomly during an otherwise routine flight. If we know our airplane, pay attention, and perform the right cross-checks, we can avoid the confusion that comes with those erroneous indications.

Robert N. Rossier, EAA 472091, has been flying for more than 30 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.

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