Wrong Runway Takeoff Alert!

[B747-400]

Hi,

ist this feature shown here modeled in PSX as well?

I setup a flight from VHHH 25L, placed the plane on 25R, but did not get this message.

Thx
Hans

[Hardy Heinlin]

Hi,

RAAS is not an option in PSX.


|-|ardy

[B747-400]

Hi,

yes, I know. But Obet describes this in the video description not as RAAS but "One other alert fitted to the Boeing 747 is through the FMC."

So, no RAAS but FMC - that's why I wonder.

Hans

[Hardy Heinlin]

EICAS caution message ">FMC RUNWAY DIS" -- I'm seeing this option the first time.

When has this been introduced?

[B747-400]

Good question, Hardy. No idea - I hope some of our experts can jump in!

Cheers!

[Mariano]

Hardy,

Please check your email.

Best regards,

Mariano

[andrej]

Hello Hardy,

in CX FCOM 1 (37.70.1.) on several planes (FCOM1 dated 11 JUN 13 and 5 MAY 14), there is listed following message:

>FMC RUNWAY DIS


On BAW FCOM (11.60.1; dated Oct 2018 Revision 66) one can also find this message:

>FMC RUNWAY DIS


EDIT: I can not find this message in BAW FCOM (11.60.1; dated Apr 2012 Revision 53)

Hope this helps and our subject matter experts will chime in.

Cheers,

[B747-400]

Hello,

I found an entry in a PDF QRH (no Airline specified) back of Oct. 2009!



405 means a Boeing Converted Freighter with P&W engines.

The List of Effective Pages states:



I've two paper QRHs:

BA from 2019:



Virgin Atlantic from 2020 does not describe the message.

Seems either to be airline specific, or removed on that planes, who have RAAS installed.
Looking fwd to our profs chime in!

Cheers
Hans

[Hardy Heinlin]

Thank you, guys. I now see it's been introduced in Atlas Air in 2020.

What might be the alert tolerance values in distance and heading? 200 meters and 20°?


|-|ardy

[B747-400]

I asked that question on YT, let's see Cpt. Obets reply ...

Cheers

[Hardy Heinlin]

I guess that's a question for the engineers of the manufacturer ...

[Owl]

Trigger conditions as follows:

Heading > 30deg

And/Or:

Distance based error (only available if FMC has GPS updating) / Approx probability of trigger:

400 ft: 10%
600 ft: 50%
800 ft: 78%
1000 ft: 92%
1200 ft: 97%

[Hardy Heinlin]

Thank you, Owl.

Do you know what that "probability" refers to?

[Owl]

Following a test flight where it didn't alert and subsequent investigation it was found that the distance based trigger was unreliable due to "variability of an overly conservative GPS parameter used in the FMC"

Thus they established the probabilities of trigger for various distances as above.

I don't have any more detail than that unfortunately.

[Hardy Heinlin]

OK, the "%" value looks like a kind of an assumed GPS precision value.
When the quality is very low, the trigger distance is 400 ft.
When nearly perfect, it's 1200 ft.

[Owl]

It's referring to the chance of triggering based on how far you are, I know it seems very odd. Some sort of rounding problem related to GPS precision as you suggest maybe

Here's as much as I can share:

The EICAS caution message >FMC RUNWAY DIS displays when the airplane position or heading is not lined up within specified limits of the active FMC departure runway and takeoff thrust is applied. GPS updating is required to enable sensing of position errors; heading errors will trigger the message even if GPS updating is disabled or unavailable.

On a recent test flight the EICAS Caution message >FMC RUNWAY DIS failed to display when the airplane was lined up on a parallel runway approximately 1000 feet from the FMC departure runway. The message displayed correctly when first tested at this location, then failed to display on another test at the same location approximately 20 minutes later.

Subsequent investigation indicates that the lack of consistent alerting was caused by variability of an overly conservative GPS parameter used in the FMC. The following table shows the approximate probability of a valid >FMC RUNWAY DIS message being displayed at various distances from the departure runway.

400 ft: 10%
600 ft: 50%
800 ft: 78%
1000 ft: 92%
1200 ft: 97%

As an example, the parallel runways at San Francisco (KSFO) are spaced about 800 feet apart. The current system will only provide an alert about 78% of the time if takeoff power is applied when lined up on the wrong parallel runway.
The >FMC RUNWAY DIS message can also be triggered when the airplane heading differs by more than 30 degrees from the departure runway heading when takeoff power is applied. This portion of the >FMC RUNWAY DIS alert functions as intended and provides reliable alerting.

And also just to clarify it's an FMC customer option not standard fit out, although weirdly not mentioned in the retrofit brochure, I've no idea when it became available.

Owl

[Jeroen Hoppenbrouwers]

Avionics reality is always surprising.

[Hardy Heinlin]

Just to sort it out in my mind, I'm inverting the "valid" data to "invalid" data:

Situation: Aircraft is at the correct position, i.e. truly 0 feet deviation.

GPS reports deviation of 400 feet in 90% of the time.
GPS reports deviation of 600 feet in 50% of the time.
GPS reports deviation of 1200 feet in 3% of the time.

That generates a Gaussian bell curve along the diameter of a circle. Most of the hits are in the center.

So to get a 400-trigger at all, the general threshold must be less than 400. Maybe 300.

Edit:
Or the phrase "of the time" refers to the permanent time, not just to the threshold checks. In that case the threshold should be a lot higher than 1200 feet in order to avoid nuisance alerts. But that high value would be useless at KSFO. -- My brain is spinning ...

[B747-400]

Interesting reading, Owl!

If I were the engineer, I would use ILS information (if available) to x-check and verify the present position additional to GPS ... but obviously it's not that easy and modeled otherwise.

Cheers

[Jeroen Hoppenbrouwers]

The report states that there was "an overly conservative GPS parameter used". So your calculations really can be adjusted a lot as long as the general erroneous reports are roughly at the stated chances. I would not fret too much about being realistic as the system as installed clearly is flawed. If you program it as flawed as it is, we can keep a Forum thread at the ready for the next ten years.

Basically if you are within 100 ft left and right of the centerline, don't warn. You are on the runway.
If you are 200 ft away, warn with 50% chance. You may be on the taxiway. Certainly not on another runway.
If you are 400 ft away, warn with 80% chance. This does not look good no matter what you do.
If you are 600 ft or more away, warn with 100% chance. Cannot be okay, ever.

Good enough...


Hoppie