One pitot, multiple instuments

[rhkennerly]

I’m seeing a lot of glass panel add-ons lately with original stream gauges AND glass EFIS. Both the air speed gauge & the EFIS have pitot inputs on the back, but there is only one pitot tube on the plane.

I knew that you could have multiple static pressure instruments on one static port. But I thought each AS instrument needed it’s own pitot.

how does that work, or am I just wrong? Can pitot instruments be daisy-chained?

 

[murphey]

 

[ExpressJetter]

I’m seeing a lot of glass panel add-ons lately with original stream gauges AND glass EFIS. Both the air speed gauge & the EFIS have pitot inputs on the back, but there is only one pitot tube on the plane.

I knew that you could have multiple static pressure instruments on one static port. But I thought each AS instrument needed it’s own pitot.

how does that work, or am I just wrong? Can pitot instruments be daisy-chained?

This video I made might help understand how it works and see how a pitot tube is very similar to a static port as in how pressure gets measured.

 

[Bell206]

how does that work, or am I just wrong? Can pitot instruments be daisy-chained?

The only requirement I'm familiar with on pitot systems is that if the aircraft requires 2 pilots each pilot station must have its own separate pitot/static system. Also recall a few airplanes/helicopters that ran air data computers as well as a standby ASI off the same pitot tube.

 

[Bell206]

Can pitot instruments be daisy-chained?

FYI: Page 2... Yes. A quick check found there was (its canceled) a specification on your question. Six instruments was the magic number per pitot tube at one time. And given how aviation specifications go there probably is another more current one under the SAE or something equivalent.

http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-I/MIL-I-5072A_16781/

 

[rotormatic1]

Some EFIS STC’s require standby instruments to fly IFR..

 

[Clip4]

I’m seeing a lot of glass panel add-ons lately with original stream gauges AND glass EFIS. Both the air speed gauge & the EFIS have pitot inputs on the back, but there is only one pitot tube on the plane.

I knew that you could have multiple static pressure instruments on one static port. But I thought each AS instrument needed it’s own pitot.

how does that work, or am I just wrong? Can pitot instruments be daisy-chained?

Yes, the airspeed instruments are reading total pressure (dynamic+static). Air is not flowing to the instruments.

 

[rhkennerly]

Thnx, but that’s one “ram” pitot measurement & 2 static instruments. I should have been more specific. What about a round gauge & a garmin G5 or similar. Both the round & thebgarmin require pitot ram pressure, but I’m only seeing one pitot tube on the plane. Doesn’t running two pitot ram instruments halve the force of the air in a mechanical system?

 

[rhkennerly]

This video I made might help understand how it works and see how a pitot tube is very similar to a static port as in how pressure gets measured.

Thx. Nice vid. But does that mean that you can run a round airspeed &, say, a garmin g5 airspeed off of one pitot tube with no error? Both have pitot intakes on the back.

 

[rhkennerly]

Yes, the airspeed instruments are reading total pressure (dynamic+static). Air is not flowing to the instruments.

Yes, the airspeed instruments are reading total pressure (dynamic+static). Air is not flowing to the instruments.

And so there is no error in plumbing one ram pitot into two different airspeed instruments…or three?

 

[Albany Tom]

Since the airspeed is based on pressure, not flow, the only error would be caused by leaks in the system, or large displacements in the instruments, and the displacement is tiny. So the reason for requiring multiple tubes is redundancy.

 

[ExpressJetter]

Thx. Nice vid. But does that mean that you can run a round airspeed &, say, a garmin g5 airspeed off of one pitot tube with no error? Both have pitot intakes on the back.

Basically the pitot tube/system is measuring ram air pressure vs static air pressure. So as long as the system is complete without breaks, 2 guages can be on the same system. Many planes use a 2nd system for redundancy but 1 system can run both

 

[PaulMillner]

Doesn’t running two pitot ram instruments halve the force of the air in a mechanical system?

No. It's the same as plugging in two bedside lamps (one on either side of the bed). They don't cut the 110 volts in half...

Paul

 

[rhkennerly]

Since the airspeed is based on pressure, not flow, the only error would be caused by leaks in the system, or large displacements in the instruments, and the displacement is tiny. So the reason for requiring multiple tubes is redundancy.

Ah…now that makes sense! It’s one of those things that you never think about until you notice it. Then you can’t stop noticing. Then you begin to argue both sides. That settles it. Thx.