Airbus Within 6ft of the Ground nearly 1 mile Short of Runway

[alf5071h]

Uplinker,

The error appears to be incorrect QNH, which results in erroneous indicated altitude - what the pilots see.

An altitude - range table, based on threshold distance, relates the altitude which should be seen at a specific distance, thus a check of the displayed altitude at x nm would identify an indicated value which was below the required chart altitude - below flight path.

The critical points here are (1) having a chart and (2) an appropriate reference position for distance - ideally the threshold. So with an adequate distance reference, the low altitude / below flight path could be identified (but not necessarily the wrong QNH).
The above does not relate to the report of this incident; see # 149

Rad Alt points noted (value of call outs … ‘Retard’ !!!!), but this assumes that crews regularly check RA indication and that it can be related to the approach; hence my previous rule of thumb - nothing complicated. Also, hearing perception - voice input is the first sense to degrade with increasing workload.

I recall that there should be a min RA in the design of every type of approach, e.g. not less than xxx at FAF; memory fails me, so my >150 ft until visual, if I look at the RA
But what did the crew look at, see?
A lesson to be shared is how the crew identified the need to GA, awareness, understanding, and how this was actioned.


EGPWS VIII Pilots Guide Rev F, dated May 2009 - check for later versions.

“The Terrain Clearance Floor (TCF) function (enabled with TAD) enhances the basic GPWS Modes by alerting the pilot of descent below a defined “Terrain Clearance Floor” regardless of the aircraft configuration. The TCF alert is a function of the aircraft’s Radio Altitude and distance (calculated from latitude/longitude position) relative to the center of the nearest runway in the database (all runways greater than 3500 feet in length).”

Improvements by modification standard:-

“In -210-210 and later versions, the TCF alert envelope and Envelope Bias Factor are improved. … . The Envelope Bias Factor is reduced (moved closer to the runway) when higher accuracy aircraft position and runway position information is available.
This is typically 1/3 to 1 nm providing greater protection against landing short events.
With version -218-218 and later models, the envelope bias factor is reduced to 1/4 nm if runway and position data is of high integrity.
Also in -210-210 and later versions, runway selection logic is improved to better identify the destination runway. Comprehensive aircraft position and navigation information is used to evaluate proximate runways and determine the most likely destination runway for all alerting purposes.”

“In -210-210 and later versions, a Runway Field Clearance Floor feature is included. This is similar to the TCF feature except that RFCF is based on the current aircraft position and height above the destination runway, using Geometric Altitude (in lieu of Radio Altitude).”

A critical aspect of the above is the nav accuracy, and GPS geometric altitude, which should have been sufficient given the type of approach being flown. But not overlooking modification standard and database updates.

I strongly suspect that this aircraft was fitted with T2CAS, an Airbus standard at some point.
I have only evaluated that system in early development thus cannot relate to current performance, but surprising if not equivalent to EGPWS.

Another factor is the generic use of ‘TAWS’, which might mask differences in systems’ capability - regulatory issue (level playing field, ambiguity, etc - the crew will manage, will m…)

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