Hi,
the FMC knows how long the selected landing runway is; its length is also used for the runway symbol generation on the ND.
Does the FMC also know how long the selected departure runway is if the runway has a displaced takeoff threshold? Or does the FMC (and ND) also use the landing distance for the takeoff?
Since the TAKEOFF REF page shows a displaced threshold of 0000 for all runways (unless the crew enters a different value), I have always assumed the FMC database contains no displaced threshold data, and therefore always uses the landing distance for both landing and takeoff.
Conclusion: If the FMC knows the displaced takeoff threshold position, there's no need to enter a displaced threshold on the TAKEOFF REF page, as the 0000 entry already refers to this public displaced threshold, not to the landing threshold. And, if this is true, the ND runway symbol will show the full takeoff distance, not just the landing distance.
In the current PSX versions the FMC doesn't know the displaced takeoff threshold position. And an entry is required on the TAKEOFF REF page to start the IRS progress at the takeoff threshold instead of the landing threshold. (Not a problem with GPS, but that's a different topic.)
Cheers,
|-|ardy
Hi Hardy,
hope this helps ... I know for sure that the FMC NavDatabase contains the displaced threshold position (in the runway database), or more precisely, the length of the threshold displacement. (*)
I assume you are aware of p.87 in the Bulfer FMC manual? I am refering to the comment of Capt. Brian Morgan. This comment seems to indicated (a LSZH Rwy 34 departure is given as an example) that on takeoff the displaced threshold - or takeoff threshold - is shown (as origin) for the departure runway on the ND.
Likewise, the comment hints that when using a full length takeoff on a runway with a displaced threshold, the threshold displacement must be entered as a negative valued in the POS SHIFT field (when flying a non-GPS aircraft) in order to avoid a map shift, as the FMC used to updated LNAV position on pressing TOGA for those airplanes (as you know). This is because the FMC uses the displayed threshold position as a takeoff position.
Markus
(*)
More detail: the FMC runway database contains the associated airport, runway identifier, currency, runway heading (T/N), length, landing threshold (lat/long), displaced threshold (in ft), elevation, ILS identifier, ILS category and ILS heading.
Hi Markus,
yes, but what is confusing me at the moment (again) is why one needs to enter a position shift if the FMC already knows the position shift.
|-|ardy
Hi Hardy,
let me just check a few facts again.
- the FMC runway database does store the landing threshold position. I just checked at EHAM 18L (FMC database vs. a good map) - this is the position of the displaced threshold (and not the beginning of the runway).
- it also stores total length and the offset of the threshold, so it can paint the correct runway length from the displaced threshold (total length minus offset)
(this video https://www.youtube.com/watch?v=YCMMskP7ue0 (https://www.youtube.com/watch?v=YCMMskP7ue0) shows a departure from EHAM 18L. Check 1:06, I think you can clearly see that runway does start 1886 ft ahead of the aircraft. They depart full length, but on the ND you can see the runway starts at the displayed threshold (by 1886 ft)
- the Bulfer manuals says (I have verified this, see above) that the FMC stores the landing threshold position (=displaced threshold). I guess this is needed because runway position might be more important for landing.
- Consequenly, if you depart from the displaced threshold, you do not need to enter a position shift in 5R.
But why does the FMC need a position shift? I think because it does not know where you depart from. (The landing point is more or less constant, e.g. determined by the ILS glidepath). Say, if you depart from KJFK 31L, you can depart from holding point Z (-3324ft), KD (0ft) or L (1600ft). The FMC does not know about it, so you need to tell him.
Since the only coordinate in the FMC runway database is the landing threshold, you need to tell him the offset based on this reference.
Markus
Just to add one point - if you have gps operational you don't enter a POS shift value.
Peter.
So PSX models this stuff correctly, correct?
I was confused because in that other thread about returning to the origin runway a training manual stated that replacing the departure runway by the landing runway (same runway facility) will also have the advantage of showing the correct runway length on the ND. This hint implies that the runway symbol length depends on the displaced threshold for takeoff. However, that video clearly shows that's not the case.
|-|ardy
Hi Hardy,
> So PSX models this stuff correctly, correct?
I think so. I re-checked the EHAM18L case and it correctly shows the 18L landing threshold when selected as a departure runway.
> This hint implies that the runway symbol length depends on the displaced threshold for takeoff.
> However, that video clearly shows that's not the case.
I agree. If you choose the same runway facility, I expect that you'll get the same runway length.
On the other hand, if you choose the opposite runway (if without displaced threshold) you'll get the full runway length on the ND. I'm sure you are aware of that, and PSX models that correctly.
Markus
p.s.
What I don't understand is the training manual. If "returning to the origin runway", why would you need "the advantage of showing the correct runway length on the ND"? Your aiming point is the landing treshold.
The only case I can imagine where it matters is at takeoff, like in the video, IF you really the full runway lenght painted on the ND (e.g. for situational awareness at dark in fog or a typhoon).