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TTU-205 Series Product Support

TTU-205 Pressure-Temperature Test Set

This page includes support and frequently asked questions for the TTU-205 Series Pressure-Temperature Test Sets. If you have additional questions or need help troubleshooting a problem, please contact our service team.

TTU-205 Series

Does the TTU-205 and Flight Line Maintenance Test Set require Air Only Calibration?

Yes. When using Secondary Standard as a controller to calibrate the TTU-205 or Flight Line Maintenance Test Sets, ensure ONLY CLEAN DRY AIR is used. The TTU-205 transducers are density sensitive and should never be calibrated with Nitrogen (N2).

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

The TTU-205 is capable of compensating for small leaks through the use of vacuum and pressure pumps. When pressure leaks out of the system at a rate faster than the TTU-205 is able to compensate, (when there is a wide open pitot or static leak, for example), the track loss light will illuminate and the TTU-205 will go into reset mode. When this happens, open the vent slowly and allow the pressure to stabilize before reapplying pressure.

If I changed only an altimeter, why do I have to connect the pitot hose to the aircraft as well as the static hose to do the checkout?

If there are any differential pressure-sensing instruments (i.e. Mach indicators, airspeed indicators, air data computers, etc.) connected to the same static source as your altimeter you MUST connect the TTU-205 pitot hose to the aircraft before running up the static. 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). The equation for this is simply Qc = Pt – Ps

Zero knots of pitot pressure is always equal to the static pressure at any altitude. For example, let’s say your outside pressure is 30 InHg. This means that the pitot pressure for 0 knots is also 30 InHg. If you run the TTU-205 up to 10,000 feet, the Ps is reduced to 20.58 InHg and the pitot pressure required to make your airspeed indicator read 0 is also reduced to 20.58 InHg. If the TTU-205 is not connected to the pitot system, it will obviously not be able to control the pressure in the pitot system. The 30 inHg of outside pressure will be in the pitot system causing the airspeed indicator to increase as the TTU-205 altitude increases. Remember, at 10,000 feet, 200 knots is approximately 22.5 InHg. If 30 inHg is in the pitot line at 10,000 feet, there will be a Qc of nearly 10 InHg which will cause the airspeed indicator to increase to over 400 knots. At 30,000 feet (8.88 InHg) the Qc will be approximately 23 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 400. in order to avoid doing it by accident, it is important that you understand how airspeed indicators and the TTU-205 operate. Always use both pitot and static hoses when you perform maintenance with the TTU-205.

How do I know what altitude to dial into the TTU-205?

Always dial 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 altitude may be 900 feet above sea level or more.

Determine your pressure altitude prior to connecting the TTU-205 to the unit under test by placing the NORMAL/READ EXT switch to READ EXT and opening the STATIC PRESSURE vent. Allow the altitude reading to stabilize and record the reading. Set the switch back to normal and close the vent, then dial in the reading you recorded as your pressure elevation. Remember to periodically recheck your pressure altitude, as it will probably change as the day goes by.

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

The TTU-205 is capable of compensating for small leaks through the use of vacuum and pressure pumps. When pressure leaks out of the system at a rate faster than the TTU-205 is able to compensate, (when there is a wide open pitot or static leak, for example), the track loss light will illuminate and the TTU-205 will go into reset mode. When this happens, open the vent slowly and allow the pressure to stabilize before reapplying pressure.

When do I have to Auto Exercise the TTU-205?

You should perform the auto exercise whenever the test set has been turned off for more than one hour, or when power has been on for more than eight hours without changing airspeed or altitude settings.

What is the proper warm-up time for the TTU-205?

The 33D7-3-60-71 says to allow 10 minutes for the tester to warm up. You will know that the test set has warmed up enough when the READY light illuminates. The 33D7-3-60-73 goes deeper into the specific operational characteristics that warrant the warm up time. Temperature variation affects the calibration of the resonant capsule pressure transducer within the TTU-205 by altering the geometry of the capsule and elastic modules as well as the density of the air next to the capsule.

Temperature sensitivity is minimized through the use of a temperature stabilizing system that employs several heaters and thermistors to maintain the transducers at a nominal operating temperature of +68°C (154.4°F). A fast warm-up heater reduces the war-up time from 45 minutes to approximately 10 minutes. When the test set transducers reach operating temperature, a thermistor output turns off the fast warm-up heater power and turns on the READY light. A separate control heater maintains the transducer temperature after the fast warm-up heater is shut down. The READY light will usually come on in a fairly short time. On a really cold day, however, it may take the entire 10 minutes to warm-up.

Is it really necessary to always have at least 50 knots of airspeed blowing thorough the TTU-205 at all times?

No. The older TTU-205’s required this because it was possible to create a Pt pressure that was less than Ps (negative Qc). This happened because the analog controllers fluctuated too much when the static pressure was changed. Of course, they had relief valves that would open if the Ps ever exceeded Pt by more than a certain amount. There were also separate vents for pitot and static. In the event of a track loss, it was necessary to vent the pitot vent before opening the static vent to avoid negative Qc.

The lowest airspeed increment on the airspeed control was 50 knots on the TTU-205B/E model and 35 knots on the TTU-205C/E models. The 50 knots was just used as added insurance to protect the unit under test from damage that would most likely result from negative Qc.

The newer TTU-205 models have very stable digital controllers that will always keep the Pt pressure positive in relation to Ps even when venting. Some aircraft T.O.’s still say to set the airspeed to 50 knots, probably because they were written back when the B/E or C/E model TTU-205’s were in use. This is unnecessary with the D, F, G, H, and J models. If you are still using the old B/E or C/E models, then yes it is necessary.

While using the TTU-205, 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 TTU-205 works with the way an airspeed indicator or an altimeter will function. The TTU-205 automatically compensates for small leaks by using its pumps to maintain whatever airspeed or altitude you command. When you turn the leak test switch on, the pressure sensor within the test set and the unit under test are both isolated from the output of the pumps and the TTU-205 will then only monitor the pressure in 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 decease. Why is this?

This is probably because you did the leak test while the altitude was run up. As we all know, when both pitot and static leak tests are required, some technicians will run up 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. Let’s say the outside pressure is an even 30 InHg. Your aircraft T.O. instructs you to do the pitot leak check at 200 knots. This means that if your TTU-205 altitude is at field pressure elevation, the pitot pressure will be approximately 31.9 InHg (greater than the outside pressure). If you do your leak test at these settings, a leak will appear as a decrease in airspeed.

Now let’s say that your TTU-205 altitude is at 10,000 feet (20.58 InHg). This means that the same 200 knots of pitot pressure is now only about 22.5 InHg (LESS than the outside pressure). If you do your leak test with the TTU-205 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 ambient. 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 TTU-205 altitude is at or below field pressure elevation in order to maximize the pressure differential.

Can you run up 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 because the TTU-205 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 TTU-205 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 InHg. The TTU-205 is connected to the pitot system only with 200 knots applied to the system. (Remember at sea level, 200 knots is equal to about 31.9 inHg. At 10,000 feet, 200 knots is only about 22.5 inHg).

If, for whatever reason (dumb airman, etc.) the TTU-205 altitude is increased, the TTU-205 will automatically decrease to 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?

How can you use the TTU-205 to check a true airspeed indicator?

Most aircraft have some sort of true airspeed (TAS) indicating system but only a few actually have T.O.’s that explain how to compute TAS for the purposes of an operational check. Some aircraft, like the KC-135, use direct reading indicators. Others, like the MH-53J, only display TAS on the navigation system. To troubleshoot a TAS problem on any aircraft you can use a pilots dead reckoning computer. The easiest kind to use is the old type E-11 but the E6B computers are more common and are just as accurate. The E6B requires you to know your -ach number which you can get from the TTU-205 display or from the “whiz wheel” inside the lid of the TTU-205..

Is it really necessary to ohm out the power cord prior to every use?

The T.O. 33D7-3-60-71 does not specifically say that it must be done prior to every use but it does have a warning that says it is very important for the power cord to be wired properly. The 33D7-3-60-73 has a chart of "prior to use" requirements but checking the power cal be is not one of them. Some individual aircraft T.O.'s require that you ohm it out while others do not. If your power cord still has the original factory plugs, then it is probably good to go. If, however, your support section replaced the power source end of the power cord with an aircraft adapter, then it may behoove to check it before you plug it in. Don't forget that if you are using an extension cord, you will need to make sure it is wired correctly as well. Remember, the black wire on pin A to high side of 115 VAC, 400 Hz power source. The white wire on pin D to low side (neutral) of 115 VAC, 400 Hz power source. The green wire, which is connected to the connector shell, must go to earth ground. Most importantly, don't forget to properly ground the tester.

Can I use the TTU-205 in the rain?

Neither the 33D7-3-60-71 nor the 33D7-3-60-73 prohibits using the TTU-205 during inclement weather. Some individual aircraft T.O.'s prohibit this while others do not. Kollsman, the OEM, says that the TTU-205 is perfectly safe to use during inclement weather because it was designed with seals on the front panel to prevent any water leakage into the system. They do suggest, however that the test set be place in the vertical position when it is operated during inclement weather.

There is also a water separator/filter that prevents moist air and contamination from entering the servo valves. Moisture is vented out through the moisture vent on the face of the tester periodically during use. Also, it is said that there is no danger in using the TTU-205 during inclement weather (including raining) as long as it is properly grounded. The chance of injury is greater for personnel who operate a TTU-205 in these kind of weather conditions without being securely attached to ground.
TTU-205 Series Manuals
Operation & Maintenance Instructions
USAF T.O. 33D7-3-60-71

Overhaul Instructions
USAF T.O. 33D7-3-60-73

Illustrated Parts Breakdown (IPB)
USAF T.O. 33D7-3-60-74

Calibration Instructions
USAF T.O. 33K6-4-700-1

TTU-205 Series Part Numbers
Current Model: TTU-205/J
P/N: 200317658-30
NSN: 4920-01-512-7743

Model: TTU-205/H
P/N: 18910480104
NSN: 4920-01-466-1502

Model: TTU-205/G
P/N: 18910480008
NSN: 4920-00-000-0190

Model: TTU-205/F
P/N: 189104800001
NSN: 4920-01-214-2410

Model: TTU-205/D
P/N: 18910480104
P/N: 18910480004
NSN: 4920-01-141-0974
NSN: 4920-00-931-1457

Model: Flightline Matinenance Test Set
P/N: 189104600000
NSN: 4920-01-247-9723

Model: TTU-205C/E
P/N: 18910010000
NSN: 4920-00-489-9110

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