GS Capture Inhibit

[andmiz]

Hi Hardy

Is it possible to create an option to inhibit G/S capture until LOC is captured? This is a customer option that we have taken up.

[Hardy Heinlin]

Hi, I think I can create this option. Is it a new option in the Boeing world?

[Markus Vitzethum]

Hi Hardy,

> Is it a new option in the Boeing world?

no, actually it is a fairly old option (I have to admit, I thought it was already modelled in PSX. Anyway... )

I just checked the e.g. DLH AOM Vol.1. It has a revised page describing this item:



Original Page (Rev 24, dated March 3rd 1997) in the "Automatic Flight" chapter, describing the APP p/b on the MCP:

- G/S capture can occur when intercept track angle is within 80 degrees of localizer course.



Revised Page ("temp revision", dated  Nov 30th 1998)

- G/S captures when the intercept track angle is within 80 degrees of localizer course; G/S capture is inhibited until localizer captured.

with an additional note at the end of the page: "Background information: The new software version inhibits glide slope capture until localizer is captured."

Markus

[Hardy Heinlin]

Thanks, Markus.

[andrej]

Is it possible to create an option to inhibit G/S capture until LOC is captured? This is a customer option that we have taken up.

Does this add more work for you when on approach? I am specifically thinking about "descent with the glideslope" clearance? It is used at some busy airports.

What is the reason? In order to be certain that you are on a correct ILS? Or possibly one could descent with a wrong GS?

Thanks!

[Markus Vitzethum]

Andrej,

I expected it had to do with avoiding the capture of false (e.g. 9°) glideslopes. However, there is this discussion on PPRUNE

https://www.pprune.org/tech-log/605824-b737-800-a-2.html

with a link to an interesting ATSB investigation report.

https://www.atsb.gov.au/media/5259090/ao2013049_final.pdf

A Qantas 737-800 was cruising at FL390 and the crew had invadvertently selected the APP button (VOR/LOC and G/S modes armed). Just before T/D, 44 minutes after incorrect APP button push, the aircraft falsely captured the Brisbane glideslope, the G/S mode became active and the aircraft climbed. (G/S was above the aircraft.)

But this was 17 years after the option became available on the DLH 744 fleet.

The report has more details (page 25) from a Boeing document:

"The ability to capture the glideslope prior to localiser capture during autopilot and flight director
approaches has raised concerns about increased aircraft exposure to the risk of terrain or
obstruction contact. Glideslope capture before localiser capture is permitted by onboard Flight
Control Computers (FCC) which will capture and track the glideslope signal without localiser
capture. During an ILS approach with an autopilot coupled to the ILS or when manually flying
the flight director, a pilot might prematurely descend on the captured glideslope while the
aircraft is not yet positioned on the localiser course. The exposure to terrain contact is increased
when an undetected map shift is present for EFIS/FMC equipped aircraft."

Markus

[andrej]

Thank you Markus for detailed response. Interesting reading you have provided.

Best,

[Will]

From a human factors standpoint, if I were designing the system, I would not prohibit G/S capture without localizer, but I would rename the "APP" button on the MCP to be called the "G/S" button. The problem is that the "APP" label is not defined intuitively. You could press "APP" when hearing ATC tell you "cleared for the approach" which makes sense in some cases, but as we know there are many cases when you would hear "cleared for the approach" but APP would be an inappropriate mode to select.

However, if the buttons were labelled LOC and G/S, then there would always be intuitive congruence between the button and the flight director mode. Another way to say it is that if the button said G/S instead of APP, then the pilots should know that when they pressed it they would be asking the FD to follow the glideslope without any preconditions. They wouldn't have to remember the details of whether APP mode needs localizer capture or not; they would just expect the flight director to follow the glideslope.

When LOC is pressed, flight director follows the localizer.
When G/S is pressed, flight director follows the glideslope.
When both are pressed, flight director follows both.

That would be my design philosophy. :-)

[Jeroen Hoppenbrouwers]

It all goes down the drain when either button is pushed inadvertently and nobody notices the mode annunciator for another hour.


Hoppie

[andmiz]

From a CFIT avoidance perspective, being laterally established on the localiser prior to glideslope capture is a safer situation than the reverse; being established on the glideslope descending whilst outside of any protected localiser guidance. You can certainly hit terrain in that scenario.

This is following GS guidance without LOC capture, only 300ft above Hong Kong's highest peak. Procedually, and with ATC guidance a difficult location to get yourself into, but...:

[Hardy Heinlin]

The G/S capture option discussed above is now implemented in PSX 10.151:

https://aerowinx.com/board/index.php?topic=4191.0


Regards,

|-|ardy