FMS VNAV and altimeter setting.

[safetypee]

What altimeter datum is used for FMS VNAV profile approaches in large commercial aircraft?
If the crew fails to set the QNH so that the aircraft is 1000ft low, i.e. 29.92 retained for a QNH of 28.92, would all of the waypoints be offset low, and thus the VNAV profile aims approx 3nm short of the runway?
Are there any differences between FMS database generated approach profiles and manual waypoints?
Are Boeing/Airbus FMS installations similar in these respects?

[Marty-Party]

Can't speak for the Airbus, but Boeing VNAV usually gets its QNH from the Captains altimeter sub-scale setting. A cross-check is made to compare the sub-scale setting with that of the F/Os altimeter. Also the VNAV database knows the approximate transition level and flags a caution is still on STD.
Yes, if the setting is wrong (i.e. left STD set or wrong sub-scale) then the aircraft will not be at the correct alitude. Backup systems such as the rad alt, ATC radar monitoring and SOPs are designed to trap this error.
Also, there is a requirement to correct for temperature on certain approaches.

[Piltdown Man]

And don't forget that you don't just let the plane get on with the descent right down into the spuds. You ""Arm" the FMS through the Altitude Select function (located, in my aircraft on Flight Mode Panel) and you enable further descents as and when you near the vertical profile steps of the approach, assuming you have been so cleared. If ATC don't specifically clear you to an altitude, the approach plate may give some guidance as to when and where you should select QNH.

[safetypee]

MP, PM thanks,
My interest comes from considering that the so called ‘much safer’ VNAV profile approach may not be as error proof as an ILS if the QNH is mis-set.
Because the ILS GS is ground based, the glidepath is always correctly positioned above the terrain. So an altitude/range check at GS capture/FAF or at the marker, should detect an incorrect QNH setting; the aircraft will have not intercepted the GS at FAF or it will be at the wrong altitude at the marker.

A ‘baro based’ VNAV approach has no such protection due to the descent profile being defined with the aircraft QNH setting, which if incorrect could result in a hazardously low flight path. Furthermore, the procedure altitude/range checkpoints will indicate that the aircraft is on the profile, but it will not be.

Many VNAV approaches are at airports without radar, and I believe warning systems such as MSAW are also based on mode ‘C’ which is similarly affected by an error in QNH setting. Could an ATM Ppruner confirm this?
Using the Rad Alt will help, but this is only really effective at lower altitudes, i.e. on an NPA the Rad Alt should always be greater than 1000 ft before the IAF, 500ft before the FAF and greater than 250ft before MDA; I assume that VNAV profile approaches are similar, is this correct?

Specific SOPs/checks/instrument scans are required to detect a QNH error, e.g. there is little value in checking that the QNH settings agree if the initial value is wrong. The procedure should require that both crew members independently obtain and set the QNH before crosschecking. It is important to avoid the practice of one pilot obtaining the value from ATIS and telling the other what it is. This weakness is also subject to controller error in providing an incorrect value. Perhaps the best gross error check is to use the preflight destination landing forecast.

Therefore, although advanced technology has improved safety by reducing the risks of NPAs, it might, without additional crew vigilance, increase the risks of a VNAV approach in comparison with an ILS.

[Shore Guy]

Safetypee and all,

It is my understanding that altitude transmitted by Mode C is pressure altitude at 29.92, and is not affected by aircraft altimeter setting. Barometric correction for ATC is applied through their equipment. Small aircraft can have “blind” altitude reporting equipment to meet the mandates (no interaction with other aircraft equipment other than transponder).

On the subject of altitude, I am aware of one carriers use of GPS altitude as a crosscheck for altitude inputs on their EGPWS. This after a couple of escape maneuvers triggered by RA “hits” during moderate rain (radar altimeter read rain as terrain). This concept could be applied to other uses (e.g. VNAV crosscheck), but I have not heard or read of such use.

GPS altitude: http://gpsinformation.net/main/altitude.htm

[safetypee]

Thanks Shore Guy
Based on your understanding of MSAW, it implies that leaving the altimeter pressure setting at 29.92 in error (the more common QNH mistake) will not enable an MSAW alert, whilst other QNH errors might be detected.
I am surprised at my own, and presumable others (due to no other response), lack of knowledge of how this valuable safety system works, and thus its dependence on correct altimeter setting procedures.

Many FMS cannot use GPS ‘geometric altitude’ due to problems of accuracy, but it is as you say good enough to provide an EGPWS alert. I was not aware that this was in commercial service, which aircraft type(s) use it?

[Shore Guy]

After two false EGPWS escape manuevers triggered by RA "hits" in moderate/heavy rain (which, by the way resulted in panels and or counterbalances of the elevator shedding), it is my understanding that the FedEx MD-11's have been modified as such. Also, I believe the radar altimeters were modified to reduce their sensitivity.

Any FedExer's out there care to add information?


https://www.ntsb.gov/ntsb/brief2.asp?ev_id=20020604X00806&ntsbno=DCA02MA039&akey=1

http://www.aircraftone.com/aircraft/accidents/20020604X00806.asp

Previous occurence:

http://www.aircraftone.com/aircraft/accidents/20001212X18968.asp

[ITCZ]

Can't speak for the Airbus, but Boeing VNAV usually gets its QNH from the Captains altimeter sub-scale setting. A cross-check is made to compare the sub-scale setting with that of the F/Os altimeter. Also the VNAV database knows the approximate transition level and flags a caution is still on STD.

Just for interest, the kit in the B717 (which is the same kit found in MD-11 and B777 I believe) has two FMS Nav systems, (Versatile Integrated Avionics systems, VIAs) left and right, that work in a dual mode where both work out their own nav solutions and monitor the other. Each generates their own Flight Director displays, and one operates in slave and one as master, but that has nothing to do with PF/PNF or CAPT/FO, only to do with which unit powered up first at the beginning of the day's flying. Each VIA takes the baro subscale setting from its respective side. The QNH cross check is automatic -- an amber "ALT" warning is displayed on both PFD's if the CAPT and FO QNH are not identical. The autoflight system is normally engaged as AP1 taking flight management information from the Captains VIA and AP2 fed by FO FMS. Should the QNH mismatch be allowed to persist the AP will fly to and capture altitudes set on the 'side' that is controlling the flight path, or the altitude selected in the ALT SEL window at the baro setting on the side that is engaged.

I enjoy running the 717 FMS trainer for my 737-800, 747-400 and 767 friends -- it makes them jealous! ;)

[safetypee]

The QNH cross check is automatic -- an amber "ALT" warning is displayed on both PFD's if the CAPT and FO QNH are not identical.
Of course, this would not help safety if both crew left 29.92 selected instead of resetting to QNH. Does the presence of such a good system cross check reduce awareness of the need to have an independent check of the actual value that has been set?
Is this similar to taking a more relaxed view of terrain (subconsciously) because MSAW will detect hazardous low altitude; except that it won’t if 29.92 is set by mistake.
And what about having EGPWS and active windshear detection fitted, does this result in a lower level of attention to the associated hazards?
These questions follow on from my interest in ‘safer’ VNAV approaches and also that there may be some indication that mis-set QNH or similar errors are increasing (experience, rumor, hearsay); and these are the errors that go undetected or more likely unreported.

[ITCZ]

So safetypee, your question is not about an altimeter mismatch, your question is what if BOTH pilots did not set the appropriate altimeter subscale for descent and landing, yes?

[safetypee]

So safetypee, your question is not about an altimeter mismatch, your question is what if BOTH pilots did not set the appropriate altimeter subscale for descent and landing, yes?
Yes.
In addition, my question is about the hazards of incorrect altimeter setting in general, and the association with FMS profile descents in particular.
If the altimeter settings are mismatched there could be a grater probability of detection as both the scale and altimeter readings differ. However, where the subscales agree this might go unnoticed, and the even more likely situation where both altimeters remain on 29.92 is particularly hazardous with FMS profile operations and the resulting lack of MSAW backup.
From a threat and error management aspect, a mis-set altimeter subscale remains a significant threat to safe operations, and requires specific procedures to avoid or identify any error.

The questions arising from the above are that does use of advanced technology make pilots less attentive and does the industry need to review SOPs where technology could initially hide some of the threats and errors.