B737: Pitch Monitor Confidence Test

[soundlover]

Hello fellas,

I guess we all know about this test, when it comes and how it annunciates but…

In the FCOM I could find only one mention about this test in the system description of the AFDS. No mention at all in FCTM and AFM.
Here in PPrune I could not find anything about it in previous posts... Thus here I am:

Does anybody know which systems the Pitch Monitor Confidence Test will check during its test?

Is there a manual or some sort of official reference where to look up this kind of detail?

Thank you

[Too Few Stripes]

This is a good question. The same wording appears in our FCOM 2 with no further explanation. It seems to described where I would expect the ILS deviation alerting system self-test to be happening.

Anyone with anything concrete on the pitch monitor confidence test?

[Skyjob]

The Pitch Monitor Confidence Test (PMCT) checks the integrity of the dual redundant pitch fault monitors.

Extracted from AMM:

(8)
Dual Channel Localizer Capture
(a) Localizer capture occurs single channel with the A/P channel first engaged and is exactly as described for single channel.
(9)
Dual Channel LOC O/C
(a) Discussion of this mode is as described for single channel
(10)
Dual Channel Approach on Course (AOC)
(a)
The single channel description remains applicable for dual approach except that at AOC the second up engaged roll autopilot channel is enabled. When the second autopilot actuator is pressurized hydraulically the system is dualized for roll.
(b)
Roll equalization begins and the Roll Confidence Test (RTC) is initiated in each channel when the second roll channel is on line. The automatic confidence test checks the lateral accelerometer circuitry. When the test passes, the Roll Monitor Confidence Test (RMTC) for that channel is allowed to start at AOC + 23 seconds. The RMTC in each channel verifies the integrity of the dual redundant roll fault monitors and they are enabled when the tests pass. When both the PMCT and RMCT in both channels have passes, FLARE ARM is annunciated to enable the pitch and roll monitors. SINGLE CH annunciation ceases at this point.
(c)
Radio altitude less than 1500 feet initiates a programmed reduction in G/S and localizer beam gains to compensate for beam convergence. Equalization gain is also reduced by radio altitude programming to maintain constant monitor sensitivity during beam convergence. Gain programming is ratcheted so that the gains can only decrease and not increase.

(11)
Dual Channel G/S Capture and Track
(a)
The single channel discussion is also applicable for dual operation. G/S capture occurs in the single "first up" channel during dual operation.
(b)
At G/S Engage an Alpha and Flaps Confidence Test is initiated in each channel whether single or dual channel has been selected. Successful completion is required for passing of the upcoming Pitch Confidence Test. (The Alpha test runs during single channel operation but because the PTC does not run, failure of the Alpha test does not have any consequences.)
(c)
At G/S + 10 seconds, the second up pitch autopilot actuator is hydraulically
pressurized and approximately 2 seconds later, dual operation begins.

(d)
At dual pitch control, equalization between channels begins and the Pitch Confidence Test is initiated in each channel to check go around, normal acceleration, flare and trim bias circuitry. At successful test completion, the Pitch Monitor Confidence Test (PMCT) for that channel is initiated which checks the integrity of the dual redundant pitch fault monitors. When the tests pass, the pitch monitors are unbypassed enabling pitch failure detection. When both PMCT's and RMCT's in both channels have passes, FLARE ARM is annunciated indicating that all fault monitoring is enabled.

(12)
Dual Channel 800-Foot Milestone
(a)
The system must be in dual channel (i.e. second paddle engaged in CMD) at 800 feet. This point is chosen to allow the system enough time to stabilize and the automatic tests to run. If the second paddles have not been engaged by 800 feet, it is locked down.
(b)
At 800 feet in APP mode with the monitors enabled, the AP Disengage Warning lights are used as additional stab out-of-trim annunciators. These lights when tripped illuminate steady red to indicate that the stabilizer is in a mistrimmed condition and that the autopilot should be disengaged.

(13)
Dual Channel Trim Bias and Monitor Sensitivity Change
(a)
A bias is added to the elevator position input to the trim detector at 400 feet. This shifts the threshold for auto stabilizer trim to drive the stabilizer and position the elevators slightly trailing edge down. This provides a "flare spring" so at low altitudes a failure which causes an automatic autopilot disconnect allows a nose up command when the elevators return to the neutral position.
(b)
The fault monitor time delay above 60 feet is 2.5 seconds for pitch and 10 seconds and 3 seconds respectively for single and dual roll. At 60-foot radio altitude, the monitor sensitivity is decreased to 1 second for pitch and 1.7 seconds for roll approach and dual-channel roll go-around. The monitor time delay below 60 feet is 10 seconds for single channel go-around.

(14)
Flare
(a)
Automatic flare is intended only for dual channel Autoland operation (Category IIIa); however, since it is an integral part of AFCS design it is functional but not annunciated during single channel operation.
(b)
Flare engages at 42 feet of radio altitude and terminates in a sink rate at touchdown of 2.5 fps. The flare command is developed by programming altitude rate as a function of radio altitude, which results in an exponential flight path to touchdown.

[soundlover]

Thank you Skyjob!
It was exactly what I was looking for!!!!!

Thank you so much!
Best regards.

[Skyjob]

You're welcome

[sky-738]

Skyjob
Will the fault monitor disconnect ap below 60feet if there is any abnormal happen? In reference to index 13 (b)

[Skyjob]

sky-738 Any fault in the monitor should trip one autopilot in dual channel mode, yes, at any stage especially below 60ft RA, hence time has been reduced to allow pilots to act and perform a go-around.

[sky-738]

thx skyjob,it helps a lot

[Denti]

Interesting, as 60ft is 140ft below the alert height after which any single failure shouldn't lead to a disconnection.

[Skyjob]

Denti, as always I appreciate your input.

Please explain then how it is possible to have an autopilot disconnect below 140'?
Crew actions in such case are standard: execute a missed approach and analyse...

[Denti]

Of course, an autopilot failure even below Alert Height (200ft) would lead to a go around if no sufficient visual reference is established. However, would a single channel autopilot failure in a fail operative autopilot system below alert height lead to a complete autopilot failure? In fail passive mode it has to do that, but does it have to do that in fail operational mode below AH?

Honestly don't know, as the documentation leaves a lot to be desired.

[RAT 5]

Is not the 'alert height' 500'. It was at this height, if memory serves, that on B757/767 the ASA pronounced if it was a LAND 3 or LAND 2 or No Autoland. This was when you made your decision to continue an autoland or possibly conduct a manual landing if RVR's allowed; or ultimately GoRound.

[Skyjob]

As we are talking 737, the quoted text is from a fail passive aircraft, not capable of fail operational, thus not able to continue the approach and logic as described.

Alert height or AH is defaulted by Boeing at 200ft but can be modified by operators.

[Denti]

The LAND3/LAND2 indication on the fail operational 737 comes on at the same time as the flare armed indication, shortly after 1500' AGL. The alert height, at least on ours, is at 200ft and at that point the decision to land or not to land is taken. Operators could use a lower decision point, but not a higher one from what I understand, as the system logic behind the AH is fixed at 200ft on the 737.

The 737NG is available as a fail operational system able of operation with no DH and 75m RVR or 50ft RA and 125m RVR OEI (if the engine fails above 200ft). An autopilot rudder servo and automatic rollout is part of the package.

[Skyjob]

Thanks for more detailed description Denti of the Fail Operational capabilities.

Alas many carriers have not ordered this option at delivery thus are incapable and restricted to Fail Passive. Including my outfit at present (new order may have changed that, but only time will tell)

[RAT 5]

I don't get the AH at 200' and it being a Land or G/A decision point. IF you operate with CAT 1 reversion options then should anything happen that causes a No Autoland and the RVR is >550m you can adjust to CAT 1 DA (nominally 200' athr). That is why I mentioned the AH as 500' on Fail operational systems as the NO Autoland display is activated then. You have time to decide and adjust. If operating fail passive, and with CAT 1 reversion options, the decision to continue to CAT 1 DA would also need to be made before 200'. Hence even for fail passive systems an AH of 500' makes sense. There are those out there who have this SOP option. Perhaps they can tell us.
To explain, I did operate B757/767 fail operational with CAT 1 reversion, but my B737 fail passive operators did not allow it, and their AH was 500'. FLARE had to be annunciated then. If not then auto land was not possible as you were only Single CH. (SOP's). With autopilot disconnect it was G/A unless visual reference was established and RVR >300m.

[Denti]

Hmm, sounds like you used a different definition of alert height than the one we use. Just for information, on our airbus fleets the AH is at 100ft.

The reason for the land/go around decision at AH is that it is the point at which the system switches into a different state, any single failure after this point will not prevent an automatic approach, flare, touchdown and rollout. Therefore the NO AUTOLAND, NO LAND 3, LAND 2 indications are inhibited below alert height. Above alert height the same failures can lead to those indications which, depending on real visual conditions, can lead to a go around.

[RAT 5]

Hi Denti. That seems to be the case. It was technology from an era before yours, but still applicable on those and similar types. It was a height at which the a/c alerted you to its capabilities and you made a decision.