744 Forum

When I pull the circuit breakers for ADC L, ADC C and ADC R I expect the ECU to go into soft reversionary mode.
(https://www.hoppie.nl/forum/var/2023-03-29_17.52.48_ECU_744.png)
In PSX the ECU remains in the normal mode: no ALTN light, no EICAS message. When I change the ambient temp the thrust decreases as if the ECU receives data from the TAT.

Please correct me if I'm wrong.

You may be right

(https://www.hoppie.nl/forum/var/2023-03-29_19.55.22_Reversionary_Mode.GIF)

But I don't understand the flow chart. What does TAT/T12 mean? ADC-sourced TAT AND fan intake temperature (T12)?

Is it like the RB211 which has a data validation system, where fan inlet temperature (T2.0) is used by the FAFC to validate and backup ADC-sourced TAT? Once the RB211 FAFC has validated the ADC data  (by comparison), it uses the ADC data. I've lost the validation flow chart for the moment.

Does "TAT/T12" indicate the validation process on the CF6?

It seems ironic that the engine sensors (T2.0, etc) are used to validate the ADC-gathered sensor data, but then the ADC sensors are used after this.

After further reading, I see that the engine sensors can't provide a substitute for "total air pressure" (which comes from the ADCs), but can substitute all the other parameters.

The alternate light seems to come because the last valid ADC data stored in EEC memory (see flow chart) will only provide a temporary solution and engine overboost is possible if it relies on soft reversionary mode for too long.

So only hard reversion (ALTN EEC switch pushed) can provide full protection. 

I think the primary question is the wording in the text at the beginning: What does the author mean by "input failure from the ADC"? In the next sentence they use the term "valid ADC information" in the same context. The validation does not necessarily check for data existence alone; it also checks for disagreement with other reference data which may come from the engine probes.

So, is it ...

a) No ADC data

b) ADC data disagrees with engine probe data


Original text:
"In the event of an input failure from the ADC, the ECU may operate in the reversionary mode. In this mode the computer uses the last valid ADC information."

In a logically consistent way, the text might also read:
"In the event of an input failure from the ADC, the ECU may operate in the reversionary mode. In this mode the computer uses the last received input from the ADC."

Or:
"In the event of invalid ADC information, the ECU may operate in the reversionary mode. In this mode the computer uses the last valid ADC information."


The same text problem occurs in the diagram text. Is "fail" a "disagreement" or a "non-existence"?


|-|ardy


I just realized, I'm having exactly the same thoughts as John H Watson :-)

Here's another version of that flow chart

(https://www.hoppie.nl/forum/var/2023-03-29_23.48.49_EECLogic.gif)

The L/R ADC input may be dependent on ISSM switch selections.

Interestingly, with loss of "total pressure" from the ADC, the ALTN light is illuminated by the EEC sending a signal to the N2 Speed Card which illuminates the ALTN light.

Still looking for the data selection flow chart...

That includes a "Last Delta T AMB" link from the normal control box. I consider that a difference check (disagreement).

Now where does the "PT2 INPUT FAIL" signal line on this (simplified) flow chart really come from?

Each EEC receives air data from the L and R ADC on databusses via ADC switching relays. Selecting the C ADC replaces one of the two ADCs.

L (or C) ADC data is sent to Channel A of the EEC
R (or C) ADC data is sent to Channel B of the EEC

Generally....

QuoteIf an input signal is determined to be faulty or missing, the
EEC is generally able to use the input from the opposite [EEC]channel.

However, I don't understand this:

QuoteADC - TAT, P (AMBIENT).. USE ENGINE SENSORS
ADC - PT2............... SWITCH TO REVERSIONARY MODE
T12, P0................. USE CROSS CHANNEL DATA. IF BOTH SENSORS FAIL, USE LAST KNOWN DATA
N1, N2, P3, T25......... USE CROSS CHANNEL DATA. IF SENSORS DISAGREE, USE CLOSEST TO CALCULATED VALUE. BOTH SENSORS FAIL, USE CALCULATED VALUE

Does this mean that EEC Channel "A" PT2 data won't be sourced from Channel B? Why wouldn't the healthy Channel take over completely? Note that T12, P0, N1, N2, P3 & T25 are engine-mounted sensors.

PT2 = "total pressure"


The GE tries to determine which is the healthiest channel and selects that. I don't know if "healthy" means the internals of the EEC alone or also "unhealthy" data from external sources.

QuoteLOSS OF BOTH EEC CHANNELS: DRIVERS (EHSV, SOLENOID)...  SWITCH TO FAILSAFE FOR ALL FUNCTIONS, EHSV TO NULL BIAS, ENGINE SHUTDOWN

Note that all of this is taken from the GE books. The RB211 and maybe the PW will be completely different.

Just a quick recap:
(https://www.hoppie.nl/forum/var/2023-03-30_09.11.37_Schermopname_(2136).png)
(https://www.hoppie.nl/forum/var/2023-03-30_09.12.09_Schermopname_(2137).png)
So only a loss of PT2 (ADC) results in a change of reversionary mode. When the TAT data is lost, the ECU uses T12.

When PT2 is lost and the transition occurs, the ECU chooses not to use T12 data. But instead the last valid ADC data is used. (Why?)

Note: This is all from KLM

Well, OK, the "other" CF6 book says this:

"If the EEC fails to receive a valid total pressure value from either ADC or probe heat fails, the EEC operates in a soft reversionary control mode. If N2 is greater than 50 percent, as sensed by the N2 speed card, the ALTN light in the EEC control switch comes on after 10 seconds and the EICAS level C message ENG X EEC MODE appears. This message is also latched as an EICAS status message."

I'll modify that for the CF6 model in the next PSX update.


|-|

My china airlines SSM says 5 sec, but probably an airline option. Thx!
(https://www.hoppie.nl/forum/var/2023-03-30_10.28.31_Schermopname_(2367).png)

That 5 sec delay per se may be correct. There are further, general delays in the system until the message actually appears on the screen, and the total time may be 10 sec then. So that SSM doesn't necessarily disagree with my quoted text.



I'm now refreshing that part in my brain which stored the EEC stuff 12 years ago. A long time. OK, one more thing. My afore-mentioned "delta" is not a delta between sensors but simply the delta between sensor and ISA:

"The selected PT2 value is used with the difference between ambient and standard day temperature (DTAMB), and the ambient temperature (TAMB) to calculate mach number (Mn) and impact pressure (Q)."

Nevertheless, there is a disagreement check:

"The normal control mode is used if PT2 LADC and PT2 RADC are both available and valid, and agree within 0.437 psia. Probe heat must also be ON. If these conditions are not met, the EEC automatically enters a soft reversionary control mode."

Despite the agreement check, this text says that the soft reversionary mode even occurs when just either PT2 alone is unavailable. That's at least my understanding when I look at logical operators like a programmer. So when a single ADC L or R fails, and, if installed, ADC C fails as well, then both PT2 are simply no longer available.

IF PT2_LADC_avail_valid_heated AND PT2_RADC_avail_valid_heated
THEN Normal_mode
ELSE Soft_rev_mode

This also agrees with the text I quoted in my previous comment.

The EEC knows the last stored DTAMB (ISA OAT deviation). So when the engine's T12 sensor indicates temp changes, the EEC also can -- based on that DTAMB -- calculate the corresponding changes in the pressure altitude. Of course, this isn't exact. It's just an assumption.

I'm wondering what happens when the changing temp from T12 isn't caused by an altitude change but by an airspeed change. The EEC might interprete an increasing airspeed as a decreasing altitude, and vice versa. This whole stuff is probably intended for the mode change only, so that there are no thrust changes in the moment of the mode change.

There are two DTAMB values. One for the soft, one for the hard reversionary mode. The latter is the "cornerpoint" DTAMB; that's greater than the other one for the soft mode. Therefore it may cause overboosting. I'm not sure why the designers added that greater "cornerpoint" DTAMB. Maybe it assures that all engines get the same reference instead of using individual last stored DTAMB values. This may also be the reason the CF6 can keep using the autothrottle when all EECs are in hard reversionary mode (which is impossible on RR and PW).

Another confusing thing regarding the hard reversionary mode:

"The cornerpoint DTAMB value is used to calculate an assumed TAMB as altitude changes, and to calculate Mn and Q."

How can the hard reversionary mode know the altitude when the EEC has just a cornerpoint DTAMB and its own temp sensors? (No ADC data.) Does this text mean to say that the hard mode will calculate an assumed altitude (like in soft mode), and by that assumed altitude it will calculate an assumed TAMB? (Which is then used to calculate Mach number and impact pressure.)

Edit: See my further comments below.

Quote from: ASCTU744 on Thu, 30 Mar 2023 10:41So only a loss of PT2 (ADC) results in a change of reversionary mode. When the TAT data is lost, the ECU uses T12.

When PT2 is lost and the transition occurs, the ECU chooses not to use T12 data. But instead the last valid ADC data is used. (Why?)

T12 is temperature. PT2 is "total air pressure". i.e. pitot pressure with temperature compensation

Quote from: Hardy Heinlin on Thu, 30 Mar 2023 11:46"The normal control mode is used if PT2 LADC and PT2 RADC are both available and valid, and agree within 0.437 psia. Probe heat must also be ON. If these conditions are not met, the EEC automatically enters a soft reversionary control mode."

Despite the agreement check, this text says that the soft reversionary mode even occurs when just either PT2 alone is unavailable.

This now seems to make more sense. Total pressure seems to be rather critical. It must be compared with something to confirm its accuracy (L and R ADC values) and the engine cannot provide an equivalent. There is no inlet pressure sensor (unlike the RB211 which has a P2T2 probe)

Quote from: Hardy Heinlin on Thu, 30 Mar 2023 11:46I'm wondering what happens when the changing temp from T12 isn't caused by an altitude change but by an airspeed change. The EEC might interprete an increasing airspeed as a decreasing altitude, and vice versa.

It might pick up altitude changes (and approximate altitude) from its engine ambient pressure sensor. I seem to recall ambient pressure (as detected by the engine P0 sensor) is used to determine height above the runway for determining when to unlock engine trimming after takeoff (at least on the RB211).

QuoteT12 is temperature. PT2 is "total air pressure". i.e. pitot pressure with temperature compensation

Thx

Quote from: John H Watson on Thu, 30 Mar 2023 13:41It might pick up altitude changes (and approximate altitude) from its engine ambient pressure sensor. I seem to recall ambient pressure (as detected by the engine P0 sensor) is used to determine height above the runway for determining when to unlock engine trimming after takeoff (at least on the RB211).

I thought the CF6 has no ambient pressure sensor:

"The air data computers (ADCs) supply T2, P0 and PT2 to each EEC. The left ADC sends data to channel A. The right ADC sends data to channel B. Engine temperature sensors send air data to the EEC."

But then:

"Ambient pressure inputs (P0 LADC, P0 RADC, P0 CH A, and P0 CH B) are used to select a P0 value."

I didn't understand these two CH A/B "P0" anyway. I thought these CH A/B items refer to the ADC P0 as well.
So are these CH A/B P0 indeed engine probes?

QuoteI thought the CF6 has no ambient pressure sensor:

There are two holes in the CF6 EEC case which lead to P0 pressure transducers. One is used for channel A, one for channel B

(https://www.hoppie.nl/forum/var/2023-03-30_13.11.22_P0_Ports_CF6_EEC.GIF)

Ah, OK. I was missing that info in the book. I was thinking they use the word "input" here for data input, not for air input.

So what I wrote earlier about DTAMB isn't accurate. The EEC doesn't use the delta for altitude calculation but for temperature calculation (when the sensed ambient pressure changes).

Here's maybe a trivial question. But it can be a trap too :-) So better think twice -- or ten times.

Background: To calculate the command N1, the EEC uses, among other inputs, the current Mach number. We know, when the OAT increases, the Mach number increases too decreases. The EEC's job is this: At a given thrust lever angle (TLA) keep the Mach number as stable as possible (for this purpose the command N1 is allowed to vary). So when flying from a cold area into a warmer region while the TLA remains constant, the EEC will increase the command N1. It will do this even when the autothrottle is OFF. It has nothing to do with the autothrottle.

When the EEC gets no OAT data anymore due to a failure, it uses its last stored deviation from ISA OAT (15°C at MSL). Based on this stored deviation, along with some other available inputs, the EEC will calculate an assumed OAT. Based on that it will also calculate an assumed Mach number.

Example: We are in an area colder than ISA, the EEC is in normal mode and it stores an ISA deviation of -7°C. A failure occurs, the EEC goes into reversionary mode. The only available inputs now are: Pressure altitude, TAT, and 7°C ISA deviation. Based on these, the EEC computes pretty much the same Mach and OAT that were sensed before the failure. Now the aircraft enters a warmer region which deviates zero from ISA. The pressure altitude remains constant. But the EEC still thinks it's 7°C colder than ISA. At the same time, the aircraft performance decreases due to the warmer air; the airspeed decreases, the pitch increases. As we know, the TAT incorporates airspeed dependent temperature and static temperature, so the former factor now decreases with the lower airspeed, and the latter increases due to the warmer region. (A deviation of 7°C would approximately correspond to a deviation of 0.1 Mach.) Anyway, I think the EEC cannot know what caused this higher TAT; is it the former or the latter factor?

So, what N1 will the EEC command now?

(a) A higher N1 because the EEC thinks the Mach number has dropped by approximately 0.1.
(b) A lower N1 because the EEC thinks the airspeed rises due to the higher TAT, while the ISA deviation is still -7°C.
(c) No N1 change because the EEC ignores the TAT and just refers to the 7°C ISA deviation and the unchanged pressure altitude.
(d) ...


I think it's (a). That will also pretty much agree with the N1 command of the normal EEC mode.
This way the difference between modes will be small.
If it were (b), the difference would be drastic. But maybe it's (c) ...


:-)

|-|ardy

I have no input to your question.

Does anybody know why the EEC here basically assumes the role I would have given to the autothrottle? That EECs equalize thrust between engines within a few percent, great. But I would think that everything else is exactly what the autothrottle should do and not the per-engine computer.

Warmer air --> more TLA.

Before the EEC, would the flight engineer do magic on their panel to increase engine thrust a bit, or would they reach forward to push the throttles up? Or would it be a pilot's action? Or would the autothrottle do it?

Hoppie

It's primarily a thrust control, not an airspeed control. The pilot sets the TLA for a specific thrust, and the EEC maintains that thrust while the OAT may fluctuate. When the air gets thinner (warmer) the RPMs need to increase. So the EEC makes the thrust control more comfortable for the crew. The pilot or the A/T need not adjust the TLA all the time.


|-|ardy

Quote from: Jeroen Hoppenbrouwers on Sat,  8 Apr 2023 19:01That EECs equalize thrust between engines within a few percent, great. But I would think that everything else is exactly what the autothrottle should do and not the per-engine computer.

There is only one autothrottle servo on the 744 for controlling all 4 levers. You can't equalise thrust by moving the thrust levers with a single servo.

Quote from: Jeroen Hoppenbrouwers on Sat,  8 Apr 2023 19:01Before the EEC, would the flight engineer do magic on their panel to increase engine thrust a bit, or would they reach forward to push the throttles up?

It was the F/E's job to make small adjustments to the thrust levers (for thrust equalisation) in cruise and on takeoff.

Quote from: Hardy Heinlin on Sat,  8 Apr 2023 13:26So, what N1 will the EEC command now?

(a) A higher N1 because the EEC thinks the Mach number has dropped by approximately 0.1.
(b) A lower N1 because the EEC thinks the airspeed rises due to the higher TAT, while the ISA deviation is still -7°C.
(c) No N1 change because the EEC ignores the TAT and just refers to the 7°C ISA deviation and the unchanged pressure altitude.
(d) ...


I think it's (a). That will also pretty much agree with the N1 command of the normal EEC mode.
This way the difference between modes will be small.
If it were (b), the difference would be drastic. But maybe it's (c) ...

I'm with Jeroen on this one. No idea  ;D

(one single throttle servo)

Yes, I get that, and that is why I see the EEC need (limited) thrust authority among each other. But I would still intuitively think that the overall thrust comes from the throttles. I know reality is stubborn and complicated. Let's try.

When you depart from a hot or high airfield, you predict that you will need more thrust to get off the runway safely, so you set the throttles higher. How much of the calculations for this are airframe-based (wings don't work as well when hot and high) and how much is engine-based (especially with N1-based engines that don't directly measure the oompf they produce)?

Just trying to understand why the mach correction has been moved into the EEC instead of for all engines together in the autothrottle/autothrust system.

It could be, for example, because there is one autothrust computer that is the same for all engine models (GE, P&W, RR) , while each engine model has its own EEC to smooth out the differences between engine models. But given all other changes to the aircraft when you swap engine models, I doubt this.

Hoppie

I think the Mach factor is used because it nicely unifies subfactors like air compression, temperature, ambient pressure etc. -- they are used to determine the maximum N1 limit (amber line). The sensed airspeed, in this context, is relevant for engine airflow control (e.g. compressor stall prevention), not relevant for aircraft airspeed control. You can set the TLA to the max and you'll get max thrust for the current ambient conditions -- which may change. For this limit function you don't even need the FMC which calculates the THR REF values (lower than the amber line). Minimum idle is also computed (based on anti-ice and flight phase etc.). The TLA scale between min idle and max thrust is accordingly interpolated, i.e. a TLA somewhere in the middle can also vary the command N1 when the EEC varies the thrust limit.


|-|ardy

Quote from: Hardy Heinlin on Sat,  8 Apr 2023 13:26We know, when the OAT increases, the Mach number increases too.

At the same time, the aircraft performance decreases due to the warmer air; the airspeed decreases, the pitch increases.

If OAT increases, so will the speed of sound (dependent of temperature only). For the same TAS, the Mach number will then decrease.

If OAT increases, air density will decrease. Since air density is in the denominator of the TAS formula, a reduction of air density will result in an increase of TAS.

Example: aircraft (340000 kg) at FL350  OAT = -61 C  CL = 0,504  TAS = 488  M0,86  TAT = -29,6
                                                               OAT = -54 C  CL = 0,504  TAS = 496  M0,86  TAT = -21,6

Keeping TAS at 488 in OAT = -54 C:  CL = 0,52  M0,847  TAT = -22,6

I have a feeling that "increase/decrease" somehow got mixed up...

Regards,
Simon

Quote from: Hardy Heinlin on Sat,  8 Apr 2023 13:26We know, when the OAT increases, the Mach number increases too.

Sorry, I meant to write: "When the OAT increases, the speed of sound increases too."

(Of course, you're right, Simon, when this happens while the TAS is constant, the Mach number will decrease, not increase.)


Thanks,

|-|ardy

Your idea about engine limits and airflow control makes perfect sense. The experienced "Mach thrust stabilization" may be a side effect.

QuotePressure altitude, TAT, and 7°C ISA deviation

Quote(https://www.hoppie.nl/forum/var/2023-03-30_09.11.37_Schermopname_(2136).png)

When switched to ALTN (soft or hard), only P0, and last AMB T (or ISA dev). So TAT shouldn't be available at all according to my source.

Quote(https://www.hoppie.nl/forum/var/2023-03-29_23.48.49_EECLogic.gif)

I wonder how my chart says something different then yours, if we follow your chart TAT data should still be available.

So I think 'C'.

According to my training books, TAT inputs are provided by: T2 LADC, T2 RADC, T12 CHA, and T12 CHB. The latter two are sensors on the engine.

Also on the schematic you can see T12 inputs going into all three EEC modes.

In case of a total ADC failure, the only inputs you lose are the "PT" type sensors (total pressure).


Some quotes from my source:

"Each EEC channel compares: Total air temperature inputs (T2 LADC, T2 RADC, T12 CHA, and T12 CH B) to select a T2 value for calculating N1 command."

So the EEC can use its T12 to select a T2 value. That is, the EEC always has a T2.

In soft mode:
"N1 command is calculated using the assumed values for Mn, Q, TAMB, and DTAMB, and the P0, T2, TLA and bleed values."


Your KLM text is not wrong; it's just incomplete. It doesn't tell the details.

That makes a lot of things clear, thx! Does the AMM cover this in detail? I can't find any chapter doing so.

The EEC modifications and the new EEC malfunctions are now available in PSX update 10.168:

https://aerowinx.com/board/index.php/topic,4191.0.html (https://aerowinx.com/board/index.php/topic,4191.0.html)


|-|ardy