Old 744 Forum

Archived posts

ALT Capture on FMA

Page: 1

Author Post
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
I was doing the VOR 13R approach into JFK today and noticed a small difference between PSX and the real -400. At our airline, we don't have a coded approach for this runway that we can fly in LNAV/VNAV. It's one of our few destinations that we still have to do a "dive-and-drive" using the vertical speed wheel, then click off the A/T, A/P and F/D at DMYHL (MAP) and turn 80 degrees to the right to line up on final (a Kai Tak-ish maneuver).

So, I was heading inbound towards ASALT (IAF) at 4000' and started a vertical speed descent to 3000, which is the ASALT crossing altitude. We are taught to stay ahead of the game by setting the next step-down altitude in the MCP as soon as we see ALT on the FMA. So, as soon as I saw ALT as the aircraft approached 3000, I selected 1500', which is the next step-down altitude to cross CRI. But instead of leveling and maintaining 3000', the aircraft kept descending towards 1500.

I did some experimenting with this and found that the aircraft will eventually capture the altitude so that you can roll in the next lower step-down altitude without it descending through the purported captured altitude; but I'm not sure if it's a function of time or whether it just cannot be descending at all prior to setting in the next lower altitude. Maybe it's neither.

I can still fly this in FLCH, but it's not recommended to be using FLCH this close to the ground on an approach because you really have no control over the vertical speed and it could set off the sink rate (>1000 FPM descent when less than 1000AGL) on the final approach segment. If this happens and you are IMC prior to reaching 800' (MDA), it would require a go around.

Whenever you see ALT appear on the FMA during a V/S descent, this should mean the aircraft has captured the altitude that is currently set in the MCP and you should then be able to run the MCP altitude down lower and still have the aircraft stay at the initially-captured altitude. Thoughts?

Jon D.
« Last edit by emerydc8 on Mon, 20 Jul 2015 02:01:23 +0000. »
Moderator
Registered: May 2009
Posts: 5140
emerydc8 wrote
... as soon as I saw ALT as the aircraft approached 3000, I selected 1500', which is the next step-down altitude to cross CRI. But instead of leveling and maintaining 3000', the aircraft kept descending towards 1500.

It descended from 3000 to 1500 while the ALT mode was engaged?


|-|ardy
Member
Registered: Sep 2010
Posts: 155
Hardy Heinlin wrote
emerydc8 wrote
... as soon as I saw ALT as the aircraft approached 3000, I selected 1500', which is the next step-down altitude to cross CRI. But instead of leveling and maintaining 3000', the aircraft kept descending towards 1500.

It descended from 3000 to 1500 while the ALT mode was engaged?


|-|ardy

Affirmative here, too. After reaching the selected altitude, ALT HOLD mode is engaged and the VERT SPD MCP window is blanked. As soon as another altitude is set, the Queen starts descending again (or climbing). V/S is the same as previously selected (in my few tests, 1000 ft/m).

UPDATE: It seems that, when this behavior occurs, the V/S is somehow restricted to about 1200 ft/m. After a while, the A/P is is leveling off the aircraft by itself.

When the PFD ALT mode is not "framed" anymore, meaning a few seconds after ALT mode engagement, a change of the MCP altitude does not lead to climbing / descending (which is the desired behavior, I guess).

Test setup is Basic 014 - Cruising at final cruise altitude.situ
_______________
Cheers
)ennis
« Last edit by Dennis B on Mon, 20 Jul 2015 14:06:49 +0000. »
Moderator
Registered: May 2009
Posts: 5140
I thought Jon was talking about observations on the real 744. My question whether "it" descended referred to the real 744.

There are two altitude hold submodes: The one submode always acquires and holds the MCP bug which may move away from the current altitude when the outside baro pressure changes or when the pilot changes the instrument baro setting; the other submode always holds the pressure altitude (1013 hPa reference) recorded at ALT engagement which is independent of instrument baro settings and which engages when pushing the black ALT mode button while the MCP altitude is not at the current altitude, or when selecting V/S 0000 fpm (which will record the pressure altitude present at first level-off and then hold that).

During V/S mode operations, the MCP altitude becomes the target only when it hasn't been changed for at least 2 seconds. As long as it is changing by turning the knob, the V/S will not capture.


Cheers,

|-|ardy
« Last edit by Hardy Heinlin on Mon, 20 Jul 2015 18:19:10 +0000. »
Moderator
Registered: May 2009
Posts: 5140
Jon, in short words, you changed the MCP altitude within 2 seconds after ALT mode engagement and thus caused the ALT mode to continue its altitude acquire submode. This will not happen if you wait for more than 2 seconds before changing the MCP again. When you tried this approach in the big sim, did you wait for 2 seconds?

This 2 second timer also affects the altitude alert system: You will see the bold white or amber altitude frame on the PFD always only 2 seconds after changing the MCP, not earlier. The same delay is applied for the C-chord sound if your airline has it.


|-|
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Hi Hardy,

I can say with absolute certainty that once you see ALT on the FMA in the Level-D sim it has captured the MCP altitude and you can move the MCP altitude down to a lower altitude without it descending to it.

[ADDENDUM]:
Quote
When you tried this approach in the big sim, did you wait for 2 seconds?


No, we do not wait for 2 seconds (unless the pilot is not paying attention). ALT capture means that you can immediately move the MCP altitude to a lower altitude and the airplane will not follow it because it has captured the previous altitude.

I didn't get this far into the approach, but when you get to 800MSL (MDA) and you see ALT on the FMA, you have to immediately set the missed approach altitude in the MCP, which in this case is 4000'. There is very little time to set this before things get really busy in the cockpit, so it's done as soon as you see ALT on the FMA. Otherwise, in the event of a go-around, you still have 800' set in the MCP and it could cause problems.


Jon D.
« Last edit by emerydc8 on Mon, 20 Jul 2015 20:15:25 +0000. »
Moderator
Registered: May 2009
Posts: 5140
I just checked PSX 10.0.0 and there it will level off at the initially selected MCP altitude even if the MCP altitude is further changed within 2 seconds.

So it looks like you discovered a side effect of a modification in a later PSX version. I wasn't aware of this. I'll try to remove it in the next update.


Cheers,

|-|ardy
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Thanks, Hardy!

Jon D.

Page: 1

Old 744 Forum is powered by UseBB 1 Forum Software