Do pitot tubes account for reduced pressure at altitude?

Question

Do pitot tubes account for reduced pressure at altitude?

Does indicated airspeed reduce with altitude? If so, how much?

Is it different for small planes vs big commercial airliners?

Answer 1

A pitot tube measures the ram airflow entering the pitot tube which is aligned with the direction of flight. The difference between static air pressure (from a hole usually located at a perpendicular position to the direction of flight, and unaffected by ram airflow pressure) and the ram airflow is then used to determine flow velocity. As a whole, this system is called the pitot-static system.

Do pitot tubes account for reduced pressure at altitude?

As the aircraft gains altitude the decreased pressure is accounted for in that the airspeed indicator continues to show the actual "indicated" airspeed by measuring the difference between the static pressure and ram airflow pressure entering the pitot tube (regardless of the altitude).

This results in an accurate "indicated" airspeed but a higher "true" airspeed as the aircraft climbs.

Does indicated airspeed reduce with altitude? If so, how much?

It's probably better to look at how much "true" airspeed changes with an increase in altitude given a constant "indicated" airspeed. There are a few factors that influence the change but a reasonable estimate is for every 1000 feet of altitude (given a constant "indicated" airspeed) the "true" airspeed will increase by about 2%.

Is it different for small planes vs big commercial airliners?

The basic principles described above regarding the pitot system are the same for airline type aircraft and small general aviation aircraft.

Answer 2

No, pitot tubes DO NOT account for reduced pressure at altitude. The indicated airspeed from the pitot tube is only equal to true airspeed at sea level on a standard day. (15C and 29.92” pressure)

As soon as you start to climb in altitude, the indicated airspeed reading will have an ERROR. The indicated airspeed from the pitot tube will be less than the actual true airspeed, and the higher you go, the more the error will be.

BUT, this error can be advantageous in regards to the stall speed of the aircraft. The stall speed may be 60 knots true airspeed at sea level and 100 knots true airspeed at altitude due to the less dense air. The pitot tube indicated stall speed at altitude will still be 60 knots “indicated” due to the density change.

So when it comes to stall speed, the pitot tube indicated airspeed does seem to “account for altitude”, but it is not intended to do so.

Indicated pitot tube airspeed decreases from true airspeed roughly 2% for every 1,000 feet increase in altitude. It is not really correct to say true airspeed increases with altitude. It is really the indicated airspeed that decreases with altitude.

A Cessna 172 aircraft will cruise at 113 knots at 2,000’ on 72% power, but 72% power at 10,000’ will yield 121 knots. So you could also say that true airspeed at a particular power setting increases with altitude because there is less drag at altitude.

Large aircraft and small aircraft both have the same error when it comes to indicated vs true airspeed.

Answer 3

No, the simple tube does not. However instrumentation may map the tube output according to a conceptual surface of multiple dimensions.

This is simple air pressure, there is no hot wire involved or MAF metering or choke sets applied to simple tubes. No hot wire, but there are heated and regulated tubes, a heat differential may be taken into account to estimate the mass air flow or density of the air.

Modern instrumentation may take into account temperatures, altitude, and use a moving sum of readings to smooth shocks or anomalies.

Read the documentation for the parts concerned.