bubbers44

OK465, Thanks, because I thought I would get beat up on my post. We old timers used a FD as a guide if it was going where we wanted to go. If not just disregard and go the way you want to go. Worked fine for years. Pretend you are a 727 and fly as you want, it works great.

I even did it with a check airman out of SJO in a 757 on my initial international captain check. He had it so screwed up after our new clearance I was in a 30 degree right climbing turn and he was in a descending dive on the FD. I ignored it and eventually he caught up. I did it right, he was trying to make automation do it.

CONF iture

Interesting.

For the FDs to reappear, if I am correct, they need 2 airspeed indications in agreement. As both recorded airspeed indications were different, probably the unrecorded one was in agreement with one of the recorded. At that time, both recorded airspeeds were below VLS, one slightly above 200, the other one around 100, and the VLS was around 240 if I get it right from the QRH.

Now, the automatic speed protection you mention applies if VLS is approached from above, but would it apply if the airspeed is already and suddenly well below VLS ?

Also, such automatic speed protection applies for a climb scenario. Would the FDs command a temporary descent in order to get back to VLS ?

A33Zab

Correct, FD needs 2 airspeeds. (FE for speedtargets calculations only 1)

@ 02:10:42 FD was transitonary available; IMO ADR2 was increasing and crossing the ADR 3 airspeed (~transitonary).
@ 02:10:47 it was consistent with ADR1 and FD returned.

I interpolated Vs1G (205t, FL350) ~ 185kts, VLS = 1.23Vs1g ~ 225kts.
anyway both airspeeds below VLS.

BEA didn't have an answer in IR#3 either:

Quote:

4.3 Recommendations relating to Flight Recorders

Quote:

Analysis of the FDR parameters and audition of the CVR provide
information that is essential to an understanding of the event.
However, it is difficult to reconstruct the indications that were available to
the crew on their instrument panel, especially the instructions given by
the Flight Director crossbars when they reappear.

They will know about now!

The latest FCPC mod 2K?* / software may be a result of this.

Ref. AD 2011-0199R1 and AD2010-0271

HazelNuts39

Agreed.

Sorry for nitpicking, but just to ensure that we work from the same basis:
At 02:10:49 ISIS altitude= 37500 ft, ADR1 airspeed=216 kCAS, M= 0.68. Then Vs1g=200 kCAS and Vls=1.23 Vs1g= 246 kCAS.

A33Zab

Not considered as nitpicking, thx for rectifying my wrong interpolation.

246kts accepted as VLS....

DozyWannabe

Sorry PJ2 - it's just that having been told I'm misinformed and know nothing of the protections, I felt we'd better make things as exact as possible!

OK465

CONF: This is as you say definitely interesting.

The specific failure mode that put you into alternate law will determine whether VLS is computed and displayed or not.

From simulations (using manual deselection & reselection of the FD's), if the failure mode does not preclude computation and display of VLS on the PFD with an operative FD then the following appears to apply:

1. If above VLS and decelerating when the FD's are reselected, the FMA vertical mode will default to VS and display the current VS and the FD pitch bar will command that specific VS (i.e. +1500) until such time that VLS is reached and then command nose down (decrease in pitch) to honor VLS. The FMA annuciation will remain VS with the displayed command (i.e. +1500) that was in effect when the FD was reselected even though the pitch bar is honoring VLS.

2. If below VLS when the FD's are reselected, and VLS computations available, the pitch bar will immediately command nose down (decrease in pitch) to honor VLS, but the FMA will still default to VS and display the specific rate in effect at the time the FD's were reselected (i.e. +400, etc.)

***************************************

If the failure mode responsible for alternate law precludes computation and display of VLS (i.e. VSW only zipper with flap position sensor problems and VLS dashes on the PERF page), then the following appears to apply:

1. Regardless of whether above or below the actual but uncomputed and undisplayed VLS, when the FD is reselected, the system will default to VS and the current rate (i.e. +1500) and the pitch bar will command that rate right into the stall, if followed.

*******************************************

I would guess that recovery from actual dual or triple ADR problems that result in 'latched' ALT 2 would result in no VLS computation or display but I'm not positive. Maybe you or A33Zab would know.

Simulations that allow for dual or triple ADR failure are directed at the failure remaining in effect until landing so that if the ADR's are 'revived' the aircraft returns to normal, not latched ALT 2.

Possibly a new simulation requirement? I'm always in the market for good tech info.

Lyman

"Also, such automatic speed protection applies for a climb scenario. Would the FDs command a temporary descent in order to get back to VLS ?"....

tantalizing.....

I see "Zipper" (5000fpm descent), " FD commanded descent" (PF's pull to defeat), and FD commanded ascent......

What does this mean to a layman?

OK465

Lyman/Layman:

With A/P disengaged, FD's do not cause the aircraft to 'do' anything, don't require 'defeating', and should be deselected when unreliable.

Different 'zipper'.

DozyWannabe

More to the point, after a UAS/autopilot disconnect incident the priority should be to just keep the thing stable. In cruise the FD, being a visual guide as to how autoflight would follow the FMS programming, is only really useful for navigation and to a lesser degree maintaining flight level. Like autopilot, the FD cannot aid a recovery.

In short, Aviate, Navigate, Communicate - in that order. The FD only really applies to the second of these.

HazelNuts39

CONF iture:

Same graph, with vertical speed and FD availability added.

HazelNuts39

The failure mode that caused reversion to Alternate2 law is detailed in IR#1 pp. 55-56 and IR#3 p. 40.

FCOM 1.27.30 associates failure of Vs1g computation with "loss of weight, or slat/flap position".

Can we conclude that Vs1g computation was available, and therefore VLS was computed and displayed?

Lyman

OK465
Lyman/Layman:

With A/P disengaged, FD's do not cause the aircraft to 'do' anything, don't require 'defeating', and should be deselected when unreliable.

Different 'zipper'.

Thank you . I recall the Flight Directors were not turned off after UAS, as they should have been according to UAS drill, AF. (Did this drill predate 447?). So any way, if the flight directors indicated "Low", would the pilots command a climb? Were they working/accurate at the loss of auto?

Do they show the 'BIRD', indicating FlightPath?

If the STALL speed was displayed, does that mean ADR'S are back on line?

OK465

HN 39 & Lyman:

Once again an interesting question that goes to the crux of whether the FD might have coerced a momentary nose up input out of him a couple of knots above stall speed that put him 'over the edge' as the pitch attitude was nearly under control.

I don't know, but of course would like to. There's not a lot of discussion of intermittent availability of FD's as a result of intermittent ADR data available to the FMGEC in the documentation I have, nor the 'latched' ALT 2 display on the airspeed tape following recovery of an ADR.

On the surface, after recovering an ADR, it would appear that the capability to compute VLS is available, but would it be displayed, possibly...

I defer to CONF or A33Zab on that.

A33Zab

I would say YES, but VLS information is somewhat conflicting and at least not clear.

FMGEC supplies GW/Cg to FCPC for calculation of Vs1G, FCPC returns this to FMGEC for VLS calculation.

FMGEC was ready to calculate VLS but was FCPC - in ALT 2 Latched -able to produce Vs1G?

BUSS (NOT on F-GZCP!) when 3 ADR switched off & FCPC revert to ALT (x) latched) is based upon VLS so why not when 2 ADRs/FMGECs returned...

I need some time to sort this out.

mm43

Back in AF447 Thread No.6 I constructed a graphic incorporating BEA Interim Report No.3 FDR data in a similar format previously used when HazelNuts39 provided simulated data derived from the BEA's Interim Report No.2.

I have now placed additional data on the graphic, and it can be viewed here.

With reference to the FD, it is interesting to note that the THS (cyan trace) takes off as the PF appears to try and hold the pitch attitude at 15° NU during the period the FD was apparently available. In the same period the thrust has played a considerable part in maintaining the NU attitude.

CONF iture

Yes, it is remarkable how, just before the FD reappear, the situation was improving, or at least stopped deteriorating. It is more noticeable in the graph linked by mm43, just above : The pitch was reducing at 6 degrees and the AoA was getting stable around 5.

But then, the FDs are back !

The question to know if the VLS is computed and displayed is very technical, but I'm not sure it has a big influence on what the FDs are commanding at that moment, or I don't think the resulted command was to put the nose down :

What we can guess is, that the PF, from that moment, is looking for to maintain 15 degrees up, and he's pretty good at it. We know the FDs are displayed, so we may assume the FDs are suggesting that 15 degrees up attitude. I don't see the PF purposely disregarding the FD commands given to him and being that focused and successful. The rule is still : If you don't follow the FD, turn them off.

In that respect, the pertinence of both ADs posted by A33Zab just earlier is also remarkable ... We were aware of the first one dealing with the operational procedure as an OEB was published, but I was ignoring the second one. You produce a lot of relevant information. Thank you.

DozyWannabe

Whoa there, hoss.

The ADs suggest that the FDs should have extra logic to force and latch them off in the case of UAS, but it's a stretch to say that this is what happened to AF447. More likely they identified it as an issue while running the sim checks and want to close that particular potential hole in the cheese.

I still don't buy the idea of a pilot following FD in the middle of (what started as) a minor emergency situation such as this unless they have a serious scan breakdown - as I said earlier, the FD is for navigation - not aviation. The other reason is that the FD would have been taking it's information from the selected values, correct? According to the trace, none of the "Selected V/S" values during HN39's excerpt even come close to requiring a 15 degree NU climb.

Lyman

When PF says: "I've lost control of the aircraft" ............

Does he instead mean: "I've lost command of the aircraft"




??

rudderrudderrat

Hi DozyWannabe,
I think with 2 or more ADRs disagreeing, the FDs bars will be removed, and all previous FD/AP modes are "forgotten".
When 2 or more ADRs agree, the FDs will reappear (because they were never turned off) and are "synched" to present aircraft attitude in HDG and VS (whatever the aircraft was doing at that moment). The FD bars will now command that VS achieved on re-engagement.

If the aircraft increased its ROD, then presumably the FD bars will command more nose up.

Probably an excellent reason to turn them off iaw the UAS procedure.