Air Data Principles and Aircraft Pitot-Static System Troubleshooting using the ADTS-3350ER or ADTS-3300JS Air Data Test Sets

Technical training does not typically provide avionics technicians with a comprehensive overview of air data principles – and therefore questions may arise due to some confusing situations.

Below we will cover some answers to questions that are commonly asked by avionics technicians.  This article is intended to help technicians testing aircraft pitot-static systems the basic air data principles and how to use the ADTS to troubleshoot them. Some air data principles can be confusing.

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If I only changed an aircraft altimeter, why would I have to connect the pitot hose as well as the static hose to perform a check?

If there are any differential pressure-sensing instruments (Mach indicator, airspeed indicator, air data computer, etc.) connected to the same static source as your altimeter, you MUST connect the ADTS pitot hose to the aircraft before running up the static channel.

Remember that the airspeed indication is a function of differential pressure or Qc. Qc is the difference between Pitot pressure (Pt) and Static pressure (Ps). Qc is calculated using the following simple formula: Qc = Pt = Ps

Zero knots of pitot pressure is always equal to the static pressure at any altitude.  For example, let’s say your ambient pressure is 30.000 inHg.  This means that the pitot pressure for 0 knots is also 30.000 inHg.  If you control the ADTS up to 10,000 feet, the Ps is reduced to 20.580 inHg, and the pitot pressure required to make your airspeed indicator read 0 is also reduced to 20.580 inHg.  If the ADTS is not connected to the pitot system, it will obviously not be able to control the pressure in the pitot system.  The 30.000 inHg of outside pressure will be in the pitot system causing the airspeed indicator to increase as the ADTS altitude increases.  Remember, at 10,000 ft., 200 knots is approximately 22.500 inHg.  If 30.000 inHg is in the pitot line at 10,000 ft., there will be a Qc of nearly 10 inHg which will cause the airspeed indicator to increase to over 400 knots.  At 30,000 ft. (8.880 inHg) the Qc will be approximately 23.000 inHg which will cause the airspeed to read over 600 knots. If your airspeed or Mach indicator is not capable of going to these speeds, you will most likely damage them.

You would never intentionally apply 600 knots of pressure to an airspeed indicator that only goes to 400.  In order to avoid doing it by accident, it is important that you understand how airspeed indicators and the ADTS operate.  Always use both pitot and static hoses when you perform maintenance with the ADTS.

While using the ADTS, I saw an airspeed indicator still reading correctly with an “out of tolerance” pitot leak.  Why is this?  Are pitot leaks really that important?

Don’t confuse how the ADTS works with the way an airspeed indicator or an altimeter will function in flight.

The ADTS automatically compensates for small leaks by using its pumps to maintain whatever airspeed or altitude you command.  When you perform a leak test, the pressure sensor within the test set and the unit under test are both isolated from the output of the pumps and the ADTS will then only monitor the pressure in the lines.  If there is a leak, you will see the pitot or static pressure try to equalize with the pressure outside the lines

In flight, there are no pumps to maintain the airspeed or altitude at the proper indication.  Any leak will cause a pressure change in the pitot system and therefore an erroneous airspeed reading.

During takeoff (when the cabin is at ambient pressure), a pitot leak will cause a low airspeed reading.  As the aircraft climbs (and the cabin is pressurized), the airspeed reading will increase due to the higher cabin pressure entering the pitot line.

Altimeters will always read low if the static system leaks inside the pressurized portion of an aircraft.  The greater the leak, the more erroneous the reading.  A leak-free pitot-static system is very important to the safety of flight.

I noticed that when I do a pitot leak test, the airspeed reading will sometimes increase rather than decrease.  Why is this?

This is probably because you did the leak test while the altitude was controlling up. Typically, when both pitot and static leak tests are required, technicians may control both the pitot and static simultaneously in order to save time.  This is where the confusion starts.

Try not to think of a leak test in terms of feet or knots—think of it in inches of mercury (inHg).  Let’s say the outside pressure is 30.000 inHg.  Your technical order might instruct you to perform a pitot leak check at 200 knots.  This means that if your ADTS altitude is at field pressure elevation, the pitot pressure will be approximately 31.900 inHg (greater than the outside pressure).  If you perform your leak test at these settings, a leak will appear as a decrease in airspeed.

Now let’s say that your ADTS altitude is at 10,000 feet (20.580 inHg).  This means that the same 200 knots of pitot pressure is now only about 22.500 inHg (less than the outside pressure).  If you do your leak test with the ADTS at these settings, a pitot leak will appear as an increase in airspeed rather than a decrease because the 30 inHg on the outside of the system is leaking into the pitot system causing an increase in pressure.

This is important to know because there is an altitude where 200 knots will equal your outside air pressure.  This altitude is only about 2,000 to 4,000 feet above a

mbient.  (refer to Figure 2.)  If you do a leak test at that altitude, you will have a no-leak indication even if your hose is wide open because there is no differential pressure between the pitot system and the outside air.

You should always do your pitot leak tests while the ADTS altitude is at or below field pressure elevation in order to maximize the pressure differential

How do I know what altitude to set in the ADTS?

It’s always good practice to know the pressure altitude (ground) prior to connecting the ADTS to the unit under test, put the unit into measure mode and open the front panel ports to ambient. You can open the manual vent valve as well.  Additionally, the ADTS features a course barometric sensor that “captures” the ground altitude when the system is powered on.

Always enter in the pressure elevation.  Remember that field pressure elevation is NOT actual elevation.  This is a very common mistake that you will see a lot. If your actual elevation is 200 feet above sea level, your pressure elevation will most likely be different depending on local weather conditions.  On a cold day with a very high barometric pressure, your pressure altitude may be less than -500 feet below sea level.  On a hot day with a very low barometric pressure, your pressure altitude may be 900 feet above sea level or more.

What does it mean when you get a track loss condition?

The ADTS is capable of compensating for small leaks through the use of vacuum and pressure pumps.  When the pressure leaks out of the system at a rate faster than the ADTS is able to compensate, (when there is a wide open pitot or static leak, for example), the track loss light will illuminate and the ADTS will go into the reset mode.

When this happens, open the vent slowly and allow the pressure to stabilize before reapplying pressure.

When should I perform an auto exercise on the ADTS?

The general rule of thumb was to perform the auto exercise whenever the test set has been turned off for more than 4  or more hours, or when power has been on for more than eight hours without changing airspeed or altitude settings. Auto exercise helps to run the transducers fully over the range of the system. Note that some transducers can exhibit hysteresis from being exercised fully, so always allow 5-10 minutes for the transducers to settle after performing an auto exercise to start testing.

What is the proper warm-up time for an ADTS?

Temperature sensitivity is minimized on the ADTS-3350ER through the use of a temperature monitoring system, that measures the temperature in several areas internally. The ADTS can be ready to use in a few minutes under normal conditions.  On a really cold day, it may take the

 entire 10-15 minutes for the internal temperature to stabilize.

Can you control the airspeed without the static hose being attached to the aircraft?

You really shouldn’t.  If all you want to check is your indicated airspeed, you should still connect the static hose to the aircraft. The reason for this is that the ADTS altitude may not always be exactly at ambient pressure.  If this is the case, your airspeed readings will be a little off.

Also, if for some reason, the ADTS altitude is increased with only the pitot hose attached, you will generate a negative airspeed into the airspeed indicator, Mach indicator, air data computer, or any other differential pressure-sensing instrument connected to the system.

For example, let’s say the outside pressure is 30.000 inHg.  The ADTS is connected to the pitot system only with 200 knots applied to the system.  (Remember, at sea level, 200 knots is equal to about 31.900 inHg.  At 10,000 feet, 200 knots is only about 22.500 inHg.)

If, for whatever reason, the ADTS altitude is increased, the ADTS will automatically decrease the pitot pressure in the lines to maintain 200 knots for whatever altitude is on the test set.  Meanwhile, you still have the 30 inHg inside the static system causing the airspeed indicator to drive off scale low.  You will most certainly damage any instrument that is connected to the pitot system if you do this.

Remember that you must always maintain a pitot pressure that is equal to or greater than your static pressure.  The easiest way to do this is to always use both hoses for whatever you do.

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