One of the most common questions we get asked by passengers is how fast we’re going. Usually it is asked about takeoff or landing as it is easy to find out how fast we’re going at cruise. For that, simply look at the inflight entertainment system which gives a readout from the onboard GPS system. When I give an answer to the takeoff or landing speed, I’ll say it depends. On what you may ask? It depends on many factors, to include the weight of the aircraft, the wind, the airport elevation, the runway conditions (wet or dry) and even the terrain surrounding the airport.
Even after explaining all that, I have to give an approximate answer because our airspeed up front is given to us in knots and not the more familiar miles or kilometers per hour. A “knot” is a nautical measure of speed which means nautical miles per hour. A nautical mile is 6076 feet as opposed to a statute or “normal” mile which is 5280 feet. In ancient days, sailors would feed a rope over the side of their ship for a specified amount of time and then measure the number of knots (which had been tied into the rope at regular intervals) that had been pulled overboard. The number of knots pulled over was proportional to the speed of the ship.
Later on, a nautical mile was defined as one minute of arc along a meridian (north-south line) on a nautical chart. This made chart reading easier and was picked up by aviation as a standard navigation protocol since early overwater aviators would have to use the same charts as used for surface navigation.
That all sounds very interesting, but are we really using the GPS readout to determine our takeoff and landing speeds? No. We are not. Airplanes stay in the air by virtue of the wind moving over the wings. Not enough wind, the wing stalls and it drops like a rock. The question is how do we know how much wind is moving over the wing?
Wind Over the Wings
To determine how much wind is flowing over the wings we use an airspeed indicator which is simply a sensor connected by plastic tubing to those odd shaped pointy things you see attached to the fuselage near the front of any airliner. Those are called pitot tubes. The tip of a pitot tube has a small opening which is connected by tubing to a pressure sensor. A measure of the air pressure from the pitot tube when compared to the ambient pressure is proportional to the speed of the aircraft through the air.
Pitot tubes, in combination with static ports (which measure ambient pressure) and their related indicators, are collectively known as the pitot-static system, and constitute one of the most vital systems on any airplane. This is why you usually see so many pitot tubes on the front of airliners. They provide redundancy.
At this point you may be raising an objection: But isn’t air a compressible fluid, and wouldn’t this compressibility skew the results as, say, temperature changed or other conditions changed? Why yes, yes they would Poindexter. Move to the front row and give yourself a star.
ICE-T (Not a drink from Long Island)
Pilots of a certain age will remember the torture inflicted by their instructors by being required to perform the dreaded “ICE-T” problem using the E6B government issue “whizz wheel” circular slide rule. This usually occurred as they were struggling to realize their dream of being a jet pilot while attempting to not throw up in the flying sterno can known as the T-37 in the west Texas summer heat. ICE-T was not an exotic drink from Long Island, but rather an acronym which stood for Indicated Calibrated Equivalent True airspeed. These terms referred to an airspeed conversion from the indicated speed shown on your panel to your actual velocity through the air known as “true” airspeed.
Performing this calculation was a drawn out process using inputs such as your pressure altitude and temperature deviation (from a standard day). It was necessary because your “true” airspeed was used in navigation calculations such as time-distance-fuel determinations.
Today, of course, those calculations are all automated by an onboard computer known as the air data inertial reference unit or ADIRU. This system takes all the pitot static input data and combines it with attitude and position data from the inertial reference units (IRUs) to provide one stop shopping data supply to the pilots’ displays, the autopilot, and even the engines which use the data to optimize things like fuel burn.
Do We Have Enough Gas?
Once you know your “true” airspeed or actual velocity through the air, you need to apply your known wind correction to determine your actual velocity across the ground. This is important, because if the headwind is, say, 30 knots stronger than what you planned for, you might not have enough fuel to reach your destination. This can ruin your day on a long overwater leg.
In years gone by, flight plans would be “winded” with the latest forecast from aviation meteorologists. The plan was only as good as the forecast, and fuel needed to be closely monitored to determine if actual headwinds were greater than forecast. INS (inertial navigation) and GPS systems have greatly increased the accuracy of fuel planning as they give real time wind readouts. You instantly know if your plan was accurate.
Wind correction data input, as you might imagine, is also automated on modern transport aircraft and fed into the aircraft’s flight management system (FMS) through an automatic data upload. This system will give you a helpful INSUFFICIENT FUEL warning if it thinks you’re not going to make it. Usually this warning means that you fat-fingered your flight plan input and told the airplane that you’re going back to your origination as your destination or some similar easily rectified mistake.
Airspeed is important for reasons beyond satisfying the curiosity of aviation fans. In the immediate short term, it keeps airplanes aloft by informing pilots when they are getting slow, which is an unforgivable sin in aviation. In the long term, knowing ground speed, which is derived from airspeed plus wind inputs, lets pilots know that they will arrive at their destination with enough fuel.