Originally Posted by Derfred
The navigation and AFDS equipment delivered as standard on a 737NG would have been quite capable of delivering the aircraft to the touchdown zone on the runway, with or without visibilty.
There is a difference between the capabilities of the aircraft, the design of the approach and the FMC coding. Even an RNP 0.10 approach will have +/- 185m lateral & 125' Vertical allowed errors. Having said all that, a typical Actual Navigation Performance (ANP) is likely to have been around 0.03 to 0.05. Lateral tracking doesn't seem to be an issue in this accident. Vertical RNP error however is highly dependant upon the QNH source accuracy, and what is
actually set in the aircraft.
I found the
04 GNSS chart (looks like valid FAA info,
more details,), which looks to have the MAP at HAMAX (assuming this is the approach they were flying) at 2.2nm from the threshold. Is the approach coded to the runway giving a valid path? If no path, what vertical flight director guidance or mode is provided beyond the MAP, or are they to be switched off?
The report shows an impact point 460m short of the runway, if you add the the TDZE (10') and a threshold crossing height of (51') and solving backward for 460m and 3
°, I get a target altitude of about 140' above the impact point to be on path. Why were they 140' low at this point? Hopefully the full report will detail the reasons.
Playing the devils advocate, who is going to spend the money to have RNP approaches constructed for every approach to replace GNSS? Are all operators going to maintain the aircraft to the required certification, nav database integrity and train their crew to utilise these approaches? I suspect everyone throws their hands up and says why should it come out of my budget? This would be an enormous project to move the whole industry in lock step to these approaches. With GLS knocking on the door, everyone would likely argue to hold out for the better technology Real Soon Now
™. In other words, nothing is likely to change any time soon, we will keep bumbling along until GLS becomes ubiquitous.