Airspeed to PSI Conversion

There are many times when we have Pitot/Static leaks in lines or fittings that have to be located and replaced.

The usual practice is to isolate the line and apply pressure to it. We then use leak detector to find the problem area (presence of bubbles).

Using this method, there is always a concern of over-pressurizing the lines and splitting one open. I've used more, but ≈ 5 PSI usually is enough pressure to locate leaks.

A low pressure regulator with nitrogen can be used, but we usually just use the Pitot/Static Test Box. Using the pitot output only..... 400 Knots is a common setting.

The question arises as to what 400 Knots is equivalent to in PSI. We use a Barfield DPS1000 . The display can be changed to show different units of measure.

Knots to inches of mercury (inHg).....
  
     
     
     
  
The tester shows two values for inHg, Pt and Qc. A little digging was required to find the difference.....

Impact Pressure (Qc) is the pressure a moving stream of air produces against a surface that brings part of the moving stream to rest. It is the difference between the total pressure (Pt) and the static pressure (Ps). These pressure properties are related by the formula: Qc = Pt - Ps  (From Mensor.com )


The link above is worth a look. Needless to say, we want to reference Qc. Using a online conversion, we found the value 8.38 inHg to be a little over 4 PSI.
  
     
  
A quick PDF might help explain all this.
  
  
 
 

Dealing with static leaks seem to be a never ending battle with the aircraft I work on.

These are all "old and worn" 767's that have a traditional Air Data Computer's.

The ADC's have two inputs, Pitot (ram air pressure from the front opening of the pitot probe used for airspeed computation), and Static (static pressure taken from the side of the aircraft or directly from ports on the pitot probe body used for altitude computation).

There is a ton of metal tubing criss-crossing the front end of the aircraft that are used to feed these pressure signals to the Air Data's.

When a tube has a hole or (most often) a "T" or elbow fitting has a crack, cabin pressure enters the line and increases sensed pressure. If a pitot line is compromised, airspeed will read high. If it's a static line, the altimeter will read low.

Finding these leaks can be difficult. We disconnect the Air Data quick disconnects (which spring seals the line shut) and apply pressure into the lines. We then go around spraying lines with leak detector looking for bubbles.
  
     
     

Using airspeed at 400 knots out of the Pitot/Static test box provides about (what we were guessing was about 4 PSI differential).

We experimented on our last troubleshooting venture. I have a low pressure regulator (something like this ) that I cranked full open while connected to the test box at 400 knots.
  
     
     
  
We had a reading of around 3 PSI. This was not a scientific, but it did show we really didn't get that much pressure out of the box.

We tried the regulator on its own with compressor air and dialed it up to 8 PSI. We could have gone higher, but for our find, 8 PSI was enough.
  
   Care should be taken to not apply these higher pressures to sensors with internal mechanical bellows (Elevator Feel Computers being one), as a possible rupture could occur.
  
  
With newer aircraft..... all these lines go away. A combined Air Data and IRU unit called (of course) a ADIRU. The pitot and static ports will have a Air Data Module located near the port. This unit sends data (I'm guessing 429 data bus) to the ADIRU.