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FMS altitude input error led to autopilot glideslope drift

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Member
Registered: May 2009
Posts: 497
Just picked up the 23 July-3 August copy of Flight international.

On page 15 it has an article with the above heading. Inshore a 777 was too low on Melbourne approach after a wrong entry of the runway threshold crossing altitude in the FMS. Crew noticed and went around, so all is well.

Still, it left me with some questions. I am sort of familiar with the displaced threshold function on the 744 FMC for take off purpose. Explained in Bulfer's guide on page 7.11

Not sure how this works? Is it 777 specific. I always thought the 747 and 777 were pretty similar when it came to the FMS functionality.

The article doesn't provide much detail, it does state that captain inadvertently entered the wrong runway threshold crossing and the FO did not validate against the approach charts or relevant manuals. Where is this shown on a approach chart? I'm familiar with the various DA/DH etc, but this is new to me.

I would have thought that based on the glideslope the threshold crossing altitude is a given for a specific plane.

Anybody any more insights into this particular threshold crossing altitude functionality?

Jeroen
Moderator
Registered: May 2009
Posts: 5140
You mean the "glideslope" of a barometric altitude based VNAV approach, not the glideslope of an ILS radio station?


|-|ardy
Member
Registered: May 2009
Posts: 374
Location: LLBG
It sounds like they did an approach with VNAV and not G/S. If you enter wrong (or change) altitude constraints in the FMC, you may screw the approach. In G/S the aircraft is ridding on a radio beam so it is different. They may notice the VNAV path didn't agree with the G/S and aborted.

Cheers,
_______________
Avi Adin
LLBG
Member
Registered: May 2009
Posts: 497
The article doesnt mention much detail, but from what I gathered they were using VNAV and only when the Co pilot noticed looking at the PAPI did they realize they were too low.

So where would you enter such a thresshold crossing altitude in the FMC? Last leg?
Thanks
Jeroen
Moderator
Registered: May 2009
Posts: 5140
Was it very cold in Melbourne?
Member
Registered: May 2009
Posts: 497
Not sure when this incident took place. Im traveeling and left the magazine home. But I dont think it ever gets below zero in Melbourne?
Jeroen
Moderator
Registered: May 2009
Posts: 5140
Jeroen D wrote
So where would you enter such a thresshold crossing altitude in the FMC? Last leg?

In the leg to the runway, i.e. where the waypoint name starts with "RW...". That's not the last leg if you have missed approach legs.

If there is no runway in the legs, use the leg which has the green E/D label on the ND.


|-|ardy
« Last edit by Hardy Heinlin on Mon, 17 Aug 2015 09:31:39 +0000. »
Moderator
Registered: May 2009
Posts: 5140
Here's a situ that is prepared for a test.

Right-click on the link below, and Save Link As ... as is in your Aerowinx/Situations folder:

Melbourne RNAV Z approach Rwy 34.situ
Member
Registered: May 2009
Posts: 497
Thanks, Ill give it a try later this week when Imget home. Where do you find this thresshold attitude. I looked for it an the approach plates, but either I'm overlooking it, or not recognising it, or its not there?

Jeroen
Member
Registered: May 2009
Posts: 131
Location: Reutlingen, Germany (near EDDS)
I'm not sure whether it has been resolved already which flight was affected.

If not, here's my contribution. :) It seems like Flight International is discussing this incident.

Here is the final report by the ATSB.

Looks like the flight crew created a RWY34 runway extension placed at 2.8 NM with an altitude crossing restriction of 380' (50' AGL) placed on the extension (and NOT the runway waypoint).
See Fig.7 on p.11 of the ATSB report. I'd say a misunderstanding is quite possible with the wording used in the briefing paper. :shock:

Markus
Member
Registered: Oct 2014
Posts: 341
Threshold elevation is stated on the ground chart. It might appear on the approach plate as well, but would need to check.
Member
Registered: Jul 2009
Posts: 148
Location: Loomis California (near Sacramento)
I was surprised to see (in the report) the amount of flight time in the past 7 days for one of the pilots. I guess "30 in 7" doesn't apply?
Member
Registered: May 2009
Posts: 497
Thanks for all the responses. The ATSB report makes for interesting reading. Good to see that they did notice the deviation from the G/S.

I wondered if they should not have gone around, rather then level off and continue unitll they intercepted the G/S?

As far as I can see in the approach plate the MDH is 495' and they were flying at 500' AGL. Obviously, they had good visibility, but still. There is a reason for the MDH, things stick up.



Jeroen

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