Large aircraft have a lot of probes for gathering data. One of my favorites on the 767 is the little guy that sits beneath the captain’s windscreen.
On humid days, the aspirated total air temperature (TAT) probe puts on quite a show; spitting and sputtering a long stream of high pressure condensation.
The probe is basically a thermometer. Why all the theatrics?
Where to Find the TAT Probe
TAT probe locations vary on different aircraft models. Some aircraft have one, others utilize multiple probes.
The 757 and 767 have a single probe (new production 767s have two TAT probes). The probe on the 767-300F(ER) is located below the captain’s windscreen. On a 757-200, the probe is on the right side of the fuselage a few feet aft of the nose gear.
TAT probes on some Airbus models are on the lower fuselage, aft of the radome.
What does the probe measure?
Air temperature, of course! There are a couple types of temperature that pilots (and aircraft) use.
- Static Air Temperature (SAT) – This is the temperature of the undisturbed air around the aircraft; also referred to as outside air temperature (OAT). When standing still outside, you feel the static air temperature.
- Total Air Temperature (TAT) – When flying fast, the temperature of the air hitting the aircraft rises significantly due to compression. Cruise TAT in a jet is typically 30°C (54°F) higher than the outside air temperature.
The TAT probe accurately measures Total Air Temperature. The ship’s air data computer derives the SAT from TAT probe data.
Compressed Air Can Be HOT!
The effects of air compression in flight can present significant engineering challenges. Supersonic airliner Concorde experienced total air temperatures (and skin temperatures) as high as 127°C (260°F) on the nose of the aircraft during Mach 2 flight.
The SR-71 Blackbird (everyone’s favorite spy plane) experienced skin temperatures of 315°C (600°F) at Mach 3. Designers used titanium for the aircraft skin to handle the extreme temperatures.
How is the temperature data used?
- Total air temperature is important when monitoring fuel temperatures on long flights (fuel tank temps tend to approach TAT).
- Pilots use both TAT and SAT to help determine when to use airframe and engine anti-ice systems. My airline’s 767 procedures require the use of engine anti-ice in visible moisture (rain, snow, clouds) between +10°C TAT and -40°C SAT.
- TAT probe data is used for calculating Mach number and true airspeed which are critical for cruise flight and navigation.
- Engine thrust setting values and auto throttles require data from the TAT probe.
Why is the TAT probe aspirated?
When an aircraft is parked or taxiing slowly, air near the probe can stagnate. Without fresh air moving past the sensor, the temperature measurement can be inaccurate.
Engineers solved the problem of air stagnation by creating a method to draw fresh air into the probe.
When an engine or auxiliary power unit is running, high pressure bleed air is fed into the bottom of the probe and out exhaust holes. The bleed air reduces air pressure inside the probe. The drop in pressure draws fresh air into the probe and past the temperature sensor. It’s a clever application of Bernoulli’s Principle.
If you’re standing near the nose of a 767, or even on a jet bridge entering the cabin, you can hear the bleed air rushing through the probe. It sounds like a high pressure air hose at a gas station.
Where To View SAT & TAT
On the 757 and 767 instrument panel, TAT is always available at the top of the center EICAS display. Pilots can access static air temperature on either FMC Control Display Unit.
In the pictures below, the aircraft was cruising at 35,000 feet. Speed was Mach .79. TAT was -13°C, and the SAT was -42°C. The air hitting the aircraft had a temperature rise of 29°C due to compression.
Condensation Show
Not only can the TAT bleed air be heard, it can often be seen. When the humidity is high, the moist air exhausting from the probe can shoot a condensation plume several feet.
Resources
- UTC Aerospace Systems – Total Air Temperature Sensor brochure (pdf)
- Goodrich Sensor Systems – Total Temperature Sensors Technical Report 5755 (pdf)
Captain Hoke! Outstanding article as usual. Thank you very much for presenting such a clear explanation of an often misunderstood probe.
Thank you for reading, Captain Palleija!
( and an extra thanks for helping me put together the idea for this one. 🙂 )
The probe also has interactions with some FMS performance calculations. We have issues some days in Bogata (High & Hot) when a ground power unit exhaust is flowing to the probe during preflight preparations and the FMS thinking it’s hotter than it actually is and the FMS will not let us enter a “reduced temperature” takeoff number. You then run around trying to get that GPU moved
Hi Brad. Great comment! I read about situations where the FMS realizes it is receiving bad data due to high probe temps. I haven’t seen that (yet) but can definitely imagine that scenario being a problem.
Thanks for reading!
Excellent article, thanks for sharing
Thanks for reading, Johnny!
Captain Ken,
Excellent article as always. Learned a lot today thanks to your effort for writing this article.
Have a great day
Hi Ehsan. Thanks for reading!
The animated image of the TAT probe expelling moisture is fascinating. Gulfstreams have two of those probes, and from all appearances they are identical to the ones on the Boeing. But in more than a decade of flying these airplanes all over the world, I cannot recall ever having seen them shoot condensation out like that. I wonder if it has something to do with their location. On all the large cabin Gulfstreams (or at least the G4/G5/G6 series that I’ve flown), they’re on the lower portion of the fuselage, just about where the nose gear is located (photo). Anyway, great post!
Hi Ron,
I’m sure it’s the same (or similar model) probe. There aren’t too many companies making them.
I suspect the probe location on the 767 makes the condensation easy to notice. I see the probe dozens of times on preflight, postflight, and taxi; and it almost always has a bleed source. So when it occasionally spews mist, I see it. Humidity needs to be high – temp/dewpoint within a degree or so, which will also limit the chances of seeing the condensation.
Do the Gulfstream probes have a bleed source from the APU? On some aircraft, they won’t get bleed air until an engine is started. In that scenario, you wouldn’t see condensation on a walk-around. And it is certainly possible that installation location could play a role; maybe some sort of microclimate where it’s installed.
Thanks for the great comment and thank you for reading!
Thanks, Captain Ken. You never cease to amaze and educate me with your fascinating, insightful, well-presented and totally readable articles. Who would have thought that such a lot of remarkable facts could stem from such a small piece of kit. You always give me something new to look for whenever I fly. (Or maybe that should be in the past tense – I hope not!)
On a separate point, I look forward to hearing you on the FR24 podcast again soon – it’s been far too long since your last appearance.
Thank you, Mike!
Yes, I always enjoy being on FR24. Great guys!
Thanks for reading,
Ken
Thank you Captain Hoke! I have been unsure about how this system worked and why it would be important to the operation of the aircraft. Great information as usual!! Keep it coming for those of us “in the back”.
Thank you for the nice comment, Steve.
Thanks for reading!
Ken
Good article on the aspirated TAT probe. Just one major point missed. TAT data is used in calculating Mach Number. Thats the reason low speed aircrafts (like turbo props) don’t have them. The pilot probe is not accurate at higher altitudes where the density is very less. There the IAS is not accurate. Hence the data from TAT is used to calculate the Mach no.
Hello Paramaguru,
You are correct that TAT probe data is used to calculate Mach number. I updated the article to include this.
To clarify, the pitot static system (including IAS) is indeed accurate at high speeds and altitudes. Pitot and TAT data are used to calculate Mach. We fly Mach numbers at high altitudes because Mach buffet is usually our limiting speed on the high end.
What a wonderful explanation! Captain Hoke, appreciate for your efforts.
TAT reading’s location is very obvious. When will pilots check it during a flight?
The wonderful animation TAT sensor shows when airplane is not moving, a motive flow is created and this is also true when the plane is airborne. My question is, why it is called total air temperature? Should it be measuring the temperature at the point the local velocity is zero?
If the main purpose is for Mach, fuel temp at long haul flight or TAS, why bother to put motive air. Isn’t it TAT etc. not critical when the plane is standing still or at low speed?
Can you give us a rough numbers of what the fuel temp like during long haul flight? Thank you Captain.
The ship’s air data computer derives the Static Air Temperature from TAT probe data. This is why the probe is aspirated.
Fuel temperature on a long flight can be anywhere from a few degrees below 0C to below -30C. It depends on the speed of the aircraft and outside air temperature. Temps at altitude vary with location and season.
Thank you Captain!
“Bleed air is fed into the TAT to draw air through” just blew me away. 1st thought is “What the heck is bleed air doing wayyyyy up there at the cockpit” and then “HOW is bleed air getting there, and what else in the cockpit utilizes it?” A theory: the whole ventilation system runs on bleed air, and the cockpit has ventilation and so…duh! Or…I don’t know. What all is using bleed air in an aircraft?
Bleed air from the engines and APU is used for several systems. On the 767, APU and/or engine bleed air is used for:
– Starting the engines
– Providing head pressure to hydraulic reservoirs
– Wing leading edge slat ice protection
– Engine cowl inlet ice protection
– Aspirated TAT probe pressure
– Air conditioning & pressurization (bleed air fed to A/C packs for cooling/distribution)
– Other stuff I probably forgot
Here’s a neat one: The Douglas DC-8 didn’t have windshield wipers. Captain and FO each had a spring-loaded handle that opened a valve to direct high pressure bleed air over the windshield. Pressure was so high that rain either didn’t hit the windshield or was immediately cleared. It worked pretty well!
Some MD11 aircraft have (or had since I haven’t seen one) a cargo loading system that used bleed air as well. Think it powered the rollers at the loading door similar to the K-loaders to orient the pallets and start them forward or back.