Question for B73NG Pilots Using GBAS approach

[kimberleyEx]

Hi All.

Just wondered to the above question. When conducting a GBAS (GLS) approach, if there was a failure of both FMC's, would a GBAS approach still be possible?

Regards.

K-Ex.

[STBYRUD]

I havent ever done one on the 737, but if you lose both FMCs you will also lose your map view and all LNAV/VNAV capability. So in your response to your question I would vote for 'Definitely not!'

[Denti]

Actually, interesting question. I would think GLS approaches are unavailable, but neither the FCOM nor the QRH (FMC fail checklist) says so. There is an additional GLS fail checklist, but that only states that all ILS and non-ILS approaches are still available, which includes IAN stuff, it is used when the additional GLS fault light on the IRS panel (aft overhead) illuminates.

After all GLS is not LNAV/VNAV or IAN, it compares your (ground augmented) position in space against the ground transmitted approach geometry, no clue if that is done in the FMC or in the MMR but i would suspect it's the FMC.

[thomasray]

no , you gonna lose all the view , because it is the main function , if it goes flat you gonna loose the view .apartment homes tucson (http://www.verranoparkapts.com/)

[Denti]

Remember, GLS approaches are not database dependent as are LNAV/VNAV approaches. GLS is similar to ILS as the approach geometry is send out by the ground station. It is however often combined with low RNP segments in intermediate approach or for the missed approach segment, and those are of course database stuff.

[Avenger]

No, not possible

[STBYRUD]

Remember, GLS approaches are not database dependent as are LNAV/VNAV approaches. GLS is similar to ILS as the approach geometry is send out by the ground station. It is however often combined with low RNP segments in intermediate approach or for the missed approach segment, and those are of course database stuff. Whaaaaat? Can I get a source for that info? ;)
As far as I've understood, in airports where it is in use, like in EDDW, the GLS channel selected determines the GBAS station to be used for GPS correction and nothing more - the actual approach info still comes from the FMC....

Edit: Okay, I should check first before asking stupid questions - in any case, I guess some system has to perform the deviation checking - and I doubt its the MMR but rather the FMC. I'll shut up now - sorry folks ;)

[Denti]

The channel number selects the approach, since one GBAS station supports numerous precision approaches within a certain range (iirc up to 49 approaches within 23 NM, or was it the other way round?) there needs to be a method to select the approach in question. In EDDW it is only two approaches, one for each landing direction.

Anyway, i would love to get a reference for Avenger's information, as i said i suspect that you need a FMC for it, but i cannot find it in our documentation although we are certified for full operational use of GLS as one of very few airlines in europe and currently fly CAT III evaluation trials for it.

[kimberleyEx]

Hi Denti.

The Channel number is selected from the NAV control panel for the ILS/GLS? Then all database information for the actual approach is held in the FMC?

That was the reason for my question. The MMR (GLU-920 I think) does not have it's own database for the Final approach fix of the GBAS (GLS) approach?

So hence, a failure of both FMC's would render GLS approach selection unavailable?

From your post I think I am on the right track. Thanks

Regards.

K--Ex.

[Denti]

The thing is you do not need any onboard database for GLS approaches except for a nice picture on the NAV and Vertical Situation display. The approach path is send out by the ground station and compared on board against the actual position, same as for an ILS.

The channel number is dialed into the NAV receiver, same as a frequency, you just have to change the mode to GLS instead of VOR or ILS.

It is all in all similar to a normal ILS, you select the approach in the FMC database, and you select the corresponding channel number (frequency for ILS) on both NAV receivers.

However i do not know where the actual position computing is done and the following position comparison. GLS is tied into the same control panel as GPS and the IRUs on the after overhead panel as far as fault detection and alerting goes, but not much else is covered in our documentation.

[Avenger]

http://www.boeing.com/commercial/aeromagazine/aero_21/gnss.pdf

Although quite old, this gives some background, without the FMC it is not possible to make the appropriate selections.

[Denti]

Exactly. The approach path is constructed entirely within the FMC, all the ground station does is transmit a correction signal to the aircraft's GPS system, such that its accuracy is improved to an appropriate level.

Not true for GLS approaches. The approach that is used by the autoflight system is not database dependent, it is information send by the ground station which provides the approach and correction data. Without the correct approach geometry data (selected via the channel number in the nav receiver and then received from the ground station) you can have the GLS approach in the FMC and still not fly it.

The article linked in by Avenger is actually very very clear on that subject:

Each approach is given a unique identifier for a particular FAS, glideslope, and missed approach combination. FAS data for all approaches supported by the particular GBAS facility are transmitted to the airplane through the same high-integrity data link as the satellite range correction data (i.e., through the VDB data link). The MMRs process the pseudorange correction and FAS data to produce an ILS-like deviation indication from the final approach path. These deviations are then displayed on the pilot’s flight instruments (e.g., Primary Flight Display [PFD] ) and are used by airplane systems such as the flight guidance system (e.g., autopilot and flight director) for landing guidance.

To fly the approach all you need is the MMR, not the FMC. The approach selection in the FMC serves only the same purpose as the approach selection for an ILS. It displays it on the ND, but is not the source of navigation data used by aircraft systems except for use as an overlay approach.

The article goes on about two different selection methods, however the FMC based selection method is not approved and supported in current operational 737 GLS installations, only selection via the NAV-panel directly into the MMR is supported. However if the approach is coded into the FMC it will show the channel number (but not select it) same as it would show (but not tune) the ILS frequency for an ILS approach.

This is in line with the rest of the 737 NAV interface which does not support auto-tuning, unlike other boeing airplanes.

By the way, thanks a lot for that artice, although it was written during the prototype phase of implementing GLS approaches it explains quite a lot.

[Pub User]

Sorry Denti,I stand corrected and have removed my post to prevent the spread of misinformation.

Like you, I suspect the postion comparison is an FMC function, but cannot find any reference to confirm it.

I'll keep looking!

[Sir George Cayley]

Apologies if this is a stoopid question, but what are the minimas for a GLS approach?

Sir George Cayley

[Denti]

At the moment we are approved down to normal CAT I minima, however we do fly some trials for CAT IIIb (75m RVR no DH).

In EDDW the minima are quite a bit higher than the corresponding ILS, i guess thats mostly because the DFS is still afraid of trying new stuff, as usual.

[FlightPathOBN]

In EDDW the minima are quite a bit higher than the corresponding ILS
Do you have the chart you are using for this procedure?

This is likely due to obstacles in the missed. GBAS is for the approach, RNAV is required for the transition to the signal and for the missed.

There are some entities using the GBAS CAT III autoland, the signal is already there, the FAA hasnt cert'd anything better than CAT I....

Currently we are working on complex GLS signals that will include curved paths. That should give the FAA something to gnaw on for a while.

[Denti]

Yup, we do run autoland trials with a honeywell GBAS ground station as well, works like a charm. However currently GBAS is officially offline in EDDW and malaga because in both countries the ground station is currently running through the certification process. The DFS in germany plans to introduce GLS approaches in EDDW, EDDM and EDDF as soon as the ground station is certified and the DLR currently runs a test ground station at EDVE.

Generally GLS should support complex approach paths (curved, different descent angles in one approach etc) right out of the box, sadly our airborne equipment doesn't support that yet as the 737 approach mode cannot cope with approaches like that. Even for normal RNP AR curved approaches we have to use LNAV/VNAV instead of IAN although IAN is usually the preferred method.

EDDW GLS 27 (https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B7xVk204frwQZWZmOGZjNjQtMDBkOS00NGQwLTkyZTYtZDZlNGVkZ jUwY2Zh&hl=en&authkey=CPCYj98M), EDDW GLS 09 (https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B7xVk204frwQMGNjN2JkMDgtOGM4OS00MmMyLTg4NDktODgzYzE4M jhiNjYx&hl=en&authkey=CJLWxvgO) and for comparison EDDW ILS 09 (https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B7xVk204frwQMGM3NzAyOTItMjI4NC00N2U5LTllNWUtM2EwMTAxO TA0YmZj&hl=en&authkey=CIrKzt4B).

[FlightPathOBN]

One can certainly have some fun with GBAS approachs...with RNP transitions to a GBAS final, you really get the airspace going...

[Denti]

PRNAV or RNP 1 RNAV transitions are the norm here for ILS approaches as you can see on the linked ILS above. Should be the same for fully operational GLS approaches i guess.

[FlightPathOBN]

EDDW GLS 27, EDDW GLS 09 and for comparison EDDW ILS 09.

Oh yes...when they want to have RNP to GBAS, it appears we can get the minima down significantly...

Note the controlling Obs for 09 is 1082...I would propose a curve to final at 5nm to avoid...

[aterpster]

I don't know a lot about GLS, but I do know a fair amount about LPV. LPV has a complex path data record associated with the database for each LPV IAP. If the database "tanks," so does LPV, and for that matter much of LNAV. ILS, OTOH, is fine because it is derived from modern versions of 1945 radio receivers.

[FlightPathOBN]

I don't know a lot about GLS

then why reply to the thread?

[aterpster]

FlightPathOBN:

then why reply to the thread?

That was obvious with my answer in context.

Do I detect a bit of hostility from a "friend?"

So, sir, is GLS totally FMS indepentent?

[Denti]

Oh yes...when they want to have RNP to GBAS, it appears we can get the minima down significantly...


In this case i'm not that sure that the obstacle is the limit. The approach and go-around path of both ILS and GLS are identical and therefore it should be possible to achieve a normal ILS CAT I minimum (current certification limit for GLS) and lateron a normal CAT IIIb limit which is what we have on the ILS. Sad thing is that currently the 737 is not able to follow curved approaches in approach mode, it needs to use LNAV/VNAV for that.

So, sir, is GLS totally FMS indepentent?

I believe that was the question at the start of this thread. We do not have a definite answer, but apparently it is. The approach path geometry is part of the GBAS datastream and the onboard multimode receiver compares the current 3 dimensional position against that received in the datastream and generates an ILS look alike signal to be displayed on the cockpit display system and to be used by the autoflight system.

[FlightPathOBN]

Sorry Terpster, I was being flippant...I should have added some :rolleyes:


GBAS is independent of the FMC much like an ILS. If the aircraft didn’t have an FMC (FedEx B-727) you can fly a GLS approach as the data and correction is delivered from the MMR (as it is with an ILS approach).

The FMC offers two benefits to the GLS approach user (as it does the ILS user), ND Map sequencing and missed approach guidance if the missed approach segment is flown in LNAV.

So if you were operating an FMC equipped aircraft on a GLS approach and had a dual FMC failure, it would not impact the approach.

[FlightPathOBN]

Denti,

In this case i'm not that sure that the obstacle is the limit.

without going too far into the chart standards, I do note that the ILS plate shows an MDA, while the GLS chart shows a DA.....

[Denti]

The ILS plate charts the LOCDME as well, which still is depicted with a MDA (shortly it will be a DA as well).

[FlightPathOBN]

Denti,

If you are doing any more test flights with the GBAS, let me know, I would like to track them on STANLY.

http://operationsbasednavigation.com/wordpress/wp-content/uploads/2011/04/STANLYBremen1.jpg

[Denti]

Well, it is part of the RNAV transition which is as far as i know at least RNP 1 (our FMC flies it as RNP 0.3), however ATC usually turns you to final early which leaves the transition and makes those nice turns around the intended line. Vectors to final are the norm for airline traffic.


Sadly GLS is officially down and not usable for the time being until it is through the whole certification process. I have to add though that EDDW is one of those places we do not visit all that often anyway, other operators use it more often, lufthansa has based the CJ1s of its flying school there.