Zero Doesn’t Always Mean Nothing


by Ken Steiner

 

 Ken Steiner     
     

Editor’s Note: Ken Steiner is the Claims Manager and Asst.  Vice-President for the San Francisco office of the United States Aircraft Insurance Group (USAIG), one of the world’s largest aviation insurance companies.  He has been with USAIG for over 30 years investigating all manner of aviation accidents involving aircraft from Cessna 150’s to Boeing 747’s.  Ken is an active pilot holding ATP and CFI certificates with over 4000 flying hours.  He flies a Turbo Skylane based at San Carlos, and yes, his flying record is free of any accidents, incidents and violations!

 

It was 109 degrees on the ramp at Sacramento International. My preflight was a half-hearted walk around the plane and an oil check. I must confess my mind was more on getting the prop started to get some air circulation than it was in a detailed aircraft inspection.

At about the time I was lifting off I noticed that the air speed indicator was reading “zero”. Some errant bug probably took up residence in the pitot tube while the plane was parked overnight. I mentally kicked myself not only for my hurried pre-flight, but also for not using a pitot tube cover during my overnight stay.

The loss of the airspeed indicator did not concern me for this flight, but it did remind of an accident I had investigated a few years back. A high-time pilot in his Mooney was taking off from the Santa Monica Airport. Well into the take off roll he noticed that his air speed indicator was reading “zero”. Although everything else was operating normally, he got so concerned with this faulty reading that he decided to abort the take-off with insufficient pavement remaining. He sailed off the end of the runway causing the landing gear to collapse and did considerable damage to the plane. There was no reason to abandon the takeoff as the laws of aerodynamics and basic pilot skill will allow an airplane to fly just fine without an air speed indicator.

A more insidious aircraft accident occurred many years ago and involved a passenger jet airliner. The flight crew noticed that the airspeed was increasing during the climb-out. They were so concerned about exceeding the mach number speed limitations that they kept increasing pitch attitude and pulled back on the power in effort to reduce the ever increasing indicated speed. The aircraft eventually ran out of speed and stalled. The crash killed all on board. What the crew was unable to figure out was that the pitot tube had become blocked. The air trapped in the line expanded as altitude increased sending an increasingly higher, but false, indicated airspeed reading to the crew.

The point is, if you need to only remember one thing, remember to “fly the plane”. There is no reason to wreck a perfectly good aircraft by becoming fixated on one problem to the exclusion of fundamental airmanship. A clogged pitot tube is not an uncommon occurrence. I’ve had 3 of them in 37 years of flying. In warm weather months insects are usually the culprits. In the winter, ice can have the same effect. Without a speed reference, you may inadvertently exceed an aircraft’s structural limitations or sail off the end of the runway during landing if going too fast. Go too slow and you may fall out of the sky.

In VFR conditions, basic attitude flying and power management will serve you well. To train for this sort of an event, find a safe altitude somewhere and cover your airspeed indicator. Approximate an approach to landing with full flaps and approach power. Establish a pitch attitude that you are accustomed to seeing during an approach with a normal rate of descent on your VSI. Uncover the airspeed indicator and see the results. Now do the same in takeoff and cruise configurations to see how close you come to the target airspeed. Practice till you are consistent.

In IFR conditions it is more a matter of flying by the numbers and reverting to partial panel technique, but the best plan here is judicious use of pitot heat to avoid the problem in the first place. That includes function testing the pitot heat system during pre-flight before launching into the icy cotton.

If you have a GPS, you automatically have a back-up air speed indicator. Convert your normal indicated approach speed to a true airspeed and deduct the headwind. For example, let’s say your normal IAS approach speed is 70 knots, and you have calculated this to be a TAS of 75 knots. As the headwind is 10 knots, your approach speed on the GPS should be about 65 kts. Your GPS may also have a TAS display function that you can monitor. It’s just a reference so don’t forgo your attitude flying skills.

A functioning airspeed indicator is a nice instrument to have onboard, but if it quits, the plane will chug along just fine without it. All the pilot has to do is fly the plane. Hopefully the only time that your airspeed is reading zero is when you are on the ground.