Hi everybody.I was always wondering, why modern airliner still rely only on pitot tubes for airspeed ?Since you can receive data from anywhere and aviation weather services are accurate, why it's not possible to switch on an EFIS from differential pressure speed to GPS speed ?You take your position, heading and GS minus minute to minute weather information = True air speed. From TAS you can get CAS and so IAS.Then you would to put your plane in a safe margin between Vs and Vh.BenjaminPS : sorry if I used the wrong terms.By "Vs" I meant stall speed and "Vh" max speed.

Hi everybody.I was always wondering, why modern airliner still rely only on pitot tubes for airspeed ?Since you can receive data from anywhere and aviation weather services are accurate, why it's not possible to switch on an EFIS from differential pressure speed to GPS speed ?You take your position, heading and GS minus minute to minute weather information = True air speed. From TAS you can get CAS and so IAS.Then you would to put your plane in a safe margin between Vs and Vh.BenjaminPS : sorry if I used the wrong terms.By "Vs" I meant stall speed and "Vh" max speed.

That would not work. GPS measures groundspeed, whereas an aircraft's actual airspeed is measured relative to the airmass through which it is moving - that information can only come from a differential pressure system (i.e. a pitot tube).If an airmass is moving due east at 100 knots, and the aircraft was flying within that airmass (also due east) at a true airspeed of 200 knots, its speed over the ground would be 300 knots. (That is what the GPS would indicate). On the other hand, if the aircraft were flying due west, it's ground speed would only be 100 knots. In both cases, the airspeed indicator (corrected for altitude and temperature) would indicate a True Airspeed of 200 knots, as the pitot tube measures the speed of the aircraft through the surrounding air. It does not matter how fast an airmass may be moving relative to the ground - as long as the aircraft is suspended within that airmass, the "speed" of the airmass is zero as far as the aircraft is concerned.Jim Barrett

That would not work. GPS measures groundspeed, whereas an aircraft's actual airspeed is measured relative to the airmass through which it is moving - that information can only come from a differential pressure system (i.e. a pitot tube).

I think you didn't really get my point or not read my post.If you know your Ground speed and the current weather "including wind force and direction" than it's possible to know your TAS.It's like flying a plane north at 105kt GS at FL080, with a last minute weather information of wind at 15kt due south for FL080, the TAS will be 120kt.

I think you didn't really get my point or not read my post.If you know your Ground speed and the current weather "including wind force and direction" than it's possible to know your TAS.It's like flying a plane north at 105kt GS at FL080, with a last minute weather information of wind at 15kt due south for FL080, the TAS will be 120kt.

TAS is airspeed corrected for temperature and pressure altitude. Those two things and the indicated airspeed are all that you need to calculate TAS so why do it the other way round? Seeing as IAS is also of extreme importance and you want it DIRECTLY measured, your idea is a little convoluted and backwards. First I want to know indicated, then the rest can follow. Being dependent on a GPS for such a basic measurement would be a little goofy...You asked this, and I quote.

I was always wondering, why modern airliner still rely only on pitot tubes for airspeed ?

Yes, they do.Also Vh= Maximum speed in level flight with maximum continuous power. Not related to the aircrafts never exceed speed, Vne.

your idea is a little convoluted and backwards.

Yours too mate, hope your are not working in airplane or avionic industries.If with, known calibration, calculation, and weather awarness you can convert IAS to GS, you can also do the opposite, GS to IAS.If your pitot tube get malfunctioning, a second source for airspeed is good, isn't it ?With my idea you can get a second source with non-precision IAS and keep it between Vs and Vne

If your pitot tube get malfunctioning, a second source for airspeed is good, isn't it ?With my idea you can get a second source with non-precision IAS and keep it between Vs and Vne

Absolutely. I'm sorry if I chose my words harshly. No offense intended. But that being said, my way seems convoluted? I mean, only every other advanced aircraft capable of doing these calculations uses this method (even a GNS430). Not to mention even private pilots are required to be able to make this calculation. :( Seems like maybe your question was a "bear trap" question since no one has the right answer. :(

Yours too mate, hope your are not working in airplane or avionic industries.

Read my signature... It's there so you can know my background!

I think you didn't really get my point or not read my post.If you know your Ground speed and the current weather "including wind force and direction" than it's possible to know your TAS.It's like flying a plane north at 105kt GS at FL080, with a last minute weather information of wind at 15kt due south for FL080, the TAS will be 120kt.

Knowing the exact air motion - (direction and velocity at a precise moment in time) - would be next to impossible. Yes, satellite-based weather services can give one a general idea of upper winds, but the scale is far too coarse to derive the exact direction, the exact velocity, for the exact position and altitude that an aircraft happens to be located at a specific moment. These satellite-based systems are not real time, and they do not have that kind of resolution. They do not provide "minute to minute" weather information as you seem to believe.GPS can indeed give an aircraft's direction and velocity over the earth's surface, in real time, to a very high degree of precision - but there is no corresponding system in existence which can (externally) give an aircraft precise, real-time information as to the direction and velocity of the airmass through which it is moving at any given altitude.What you suggest would work if exact air motion could be provided - but it would be a horribly complicated way to calculate something which can far better be done with the simple and time-proven on-board pitot system, coupled with an on-board air data computer to instantaneously correct indicated airspeed for temperature and pressure.Jim Barrett

The Pitot Tube is the most reliable source for measuring exact airspeed and detecting stalls. Yes you could calibrate other equipment to get a near result but any discrepancies in those multiple equipment may lead to inaccurate speed readings and stall of an aircraft. At least a functioning Pitot Tube has no discrepancies and is accurate.As my former flight instructor used to say...."It may be old tech, but it is good tech."Would you replace your speedometer on your car that mechanically measures wheel rotation with an electronic GPS speedometer? or just go with what already works fine.

To rely on an outside source for exact wind speed and direction; you'd need an array of teathered weather baloons every 1/8 of a mile, every couple hundred feet high ..BUT.. a modern GPS that knows your ground-speed, heading, and track, can indeed deduce the current wind vector, then you could calculate TAS. I was shown how to do it on a G430, but it was kinda tedious and silly with an ASI right in front of me.I suppose it would be OK for a PIREP or something (knowing the GPS calculated wind vector), but for piloting purposes, your well honed instinct for how pitch/power=airspeed will get you through a failed ASI..As for modern jets.. I can't imagine it would be a big deal for all that on-board intelligence to come up with a TAS... ala a guy in a small airplane with a G430, and a E6B..

Considering the rather critical nature of IAS to safe pilotage, one needs to be able to react quickly to any excursions at or beyond aerodynamic limits. It is mainly for this reason that the simple pitot system remains. This an ideal example of KISS in action. :(

Considering the rather critical nature of IAS to safe pilotage, one needs to be able to react quickly to any excursions at or beyond aerodynamic limits. It is mainly for this reason that the simple pitot system remains. This an ideal example of KISS in action. :(

That's precisely what I said in an earlier post.

That's precisely what I said in an earlier post.

Of course it is. One can say the same thing in different words, after all. :( Note that "ytzpilot" also said essentially the same thing, albeit in slightly different words. The basic concept however remains the same.What zooms over the head of some folks just might hit them in the face instead if phrased in another form... :(

Just reiterating! :( Wanted to emphasize this point!

Climbing and descending could make quite a difference..........if using GPS ground speed as a baseline.L.Adamson

Climbing and descending could make quite a difference..........if using GPS ground speed as a baseline.L.Adamson

Absolutley.. and during those two events, is when knowing instant/accurate airspeed can be critical.