B787 GENX Engine

[uzintheair]

Hi Aviators,
does anyone have more information about the TAT probe sensor (T20P20) in the engine inlet ? Like to which system it sends data to etc.

[tdracer]

The GEnx doesn't have a T20/P20 sensor. No pressure sensor in the inlet - only total temperature. Inlet TAT read by and used by the FADEC, and sent to the aircraft for fault detection of the aircraft air data TAT sensors.

[uzintheair]

Ah ok great info. Many thanks. Just one more question for what the FADEC needs TAT probe data ?

[safetypee]

Was this the type of probe - FADEC interface, which initially had problems due to ice crystal icing?

[tdracer]

uzintheair

The FADEC uses TAT for several functions - including ratings calculations and scheduling the variable stator vanes (VSV).

safetypee

Not sure what 'initially had problems" you're speaking of. The TAT probes on GE (and CFM) engines are not heated - and hence not susceptible to ICI (ice crystals are an issue on heated surfaces - which melt the ice which then rapidly re-freezes). Both Pratt and Rolls have had issues with inlet probes icing up in ICI conditions - but they are heated due to the pressure sensor.
The GEnx engine core had issues with ICI - ice would form in the compressor then shed, which could flameout the engine and damage compressor blades.
We had a separate issue with the inlet TAT probe - basically the probe was not very good and we had large probe to probe variations in readings (the probe was new and unique to the GEnx). So GE changed back to the TAT probe used on the GE90 - and the original TAT probes were replaced (there was a compliance time period allowed but has long since passed).

[safetypee]

td, thanks.
The question relates to how regulatory authorities manage the period between identifying a problem and resolution by component change.

The engine issue - potential simultaneous loss, or lack of thrust control due to TAT / FADEC malfunction, tasked crews with the difficult identification of likely ICI conditions and avoidance - by a large margin.
I don't recall any adverse events in the interim period.

With an different system / aircraft affected by ICI, resulting in simultaneous loss of function, the regulator focussed on recovery; there were events with varying success depending on crew performance.

[uzintheair]

tdracer do you have any references to this to read up further ? Would be much appreciated and I wasn’t really aware that ICI is only an issue for warm surfaces.

[safetypee]

Re ‘warm’ surfaces, ( warmer than freezing ) Boeing info: https://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/AERO_Q407_article3.pdf

Also: https://www.skybrary.aero/index.php/High_Level_Ice_Crystal_Icing:_Effects_on_Engines
and
https://www.skybrary.aero/index.php/Avoidance_of_High_Altitude_Ice_Crystal_Icing_Conditions_-_FAA_Airworthiness_Directive,_No_2013-24-01_-_Potential_Implications_for_ATC

My interest was associated with -
https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/97fc012f45f25bd086257c30004be236/$FILE/2013-24-01.pdf
N.B. Crew procedures; page 7 - 8
td has explained that this is engine related, not the particular TAT probe.

Was this AD part of the fix?
https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/d81ac13eaa884b868625858c004176f8/$FILE/2020-13-04.pdf

[tdracer]

Just a short explanation of Ice Crystal Icing (ICI). Conventional icing (and most of the regulations related to icing) involves 'super cooled droplets' - i.e. droplets of liquid water that's at a temperature blow freezing. These droplets hit a cold surface (such as an aircraft) and immediately freeze. ICI is fundamentally different - it occurs when it's simply too cold for 'super cooled droplets' to exist, so the droplets freeze into crystals. ICI generally exists at higher altitudes (30k and up) and tend to form above convective weather systems (e.g. thunderstorms).
ICI is not a danger to cold surfaces - it simply bounces off. But, when ICI hits a warm surface (above freezing), it will melt. The danger is when there are high concentrations of ice crystals - the first crystals melt, but then additional crystals hit and cool the liquid water and it re-freezes - and large accumulations of ice are possible. This ice can often shed all at once, causing downstream damage to the compressor and quenching the combustor flame. We've seen ice form in compressors where the initial surface temperature was as high as 90F/32C.

Was this AD part of the fix?
https://rgl.faa.gov/Regulatory_and_G...2020-13-04.pdf (https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/d81ac13eaa884b868625858c004176f8/$FILE/2020-13-04.pdf)
Yep. GE came up with a rather clever FADEC algorithm that can detect ICI, and when ICI is detected the FADEC cycles the Variable Bleed Valves to dump ice out of the compressor before it accumulates enough to cause a problem. There was also an issue unique to the GEnx-2B engine on the 747-8 where the location of the fan support struts combines with the fan exit guide vanes to create an ice accumulation point, so there is a fix in the AD that re-clocks the exit guide vanes with respect to the struts to eliminate that ice accumulation point ( the -1B on the 787 didn't have that problem).

[Naff_Ke]

GenX engine uses T12 instead.

1. The T12 sensor supplies the fan inlet air temperature data to the electronic engine control (EEC).
2. The EEC uses the T12 sensor as backup to the airplane total air temperature (TAT).
- TAT is used for control function and power management of the engine.
- If TAT is not available from the airplane air data reference system, the EEC uses T12 data.
3. The EEC compares the compensated T12 value to the TAT provided by the airplane.
- If the airplane TAT and compensated T12 temperature agree within +/- 2.5C, the EEC uses the airplane TAT for its final selected TAT signal.
- This is the signal used for thrust management.
4. The EEC uses T12 to calculate:
- Mach
- Altitude
- Calibrated air speed (CAS)
- Standard day temperature.
5. The EEC also uses T12 for:
- Thrust scheduling
- Variable bleed valve (VBV) control
- Variable stator vane (VSV) control.

[uzintheair]

Hi Guys,
great great content. Thanks for sharing! This information you won’t find in company manuals.