For those suggesting that maybe it was a mis-set Altimeter that might have caused this crash, the normal sequence and settings would be as follows:
- Takeoff: Departure airfield QNH.
- Climbing and passing the Transition Altitude and / or cleared to a FlightLevel: 1013 (STD).
- Cruise (above transition altitude): 1013 (STD).
- Descending and passing the Transition Level / and or cleared to an Altitude: Arrival Airfield QNH.
On the B737-NG the FMC comes pre-programmed with a default Transition Altitude (
climbing) and a default Transition Level (
descending), with the respective actual altitude / level typically being airline / airfield specific.
On each pilot's EFIS Control Panel there is a multi-function button which allows that pilot to select either QNH or 1013 (STD) on their respective altimeter.
If a pilot fails to select 1013 (STD) when climbing above Transition Altitude OR fails to select QNH when descending below the Transition Level, then the pressure reference field (
at the bottom of the altimeter tape on their Primary Flight Display / PFD) will present as an amber box, i.e. to highlight that their particular altimeter is not operating in accordance with an appropriate pressure reference (
as defined within the FMC).
Furthermore, were the pilots to have different QNH's set on their respective altimeters, such that the altimeters differed by more than 200 feet (
for more than 5 seconds) then they would get a 'ALT DISAGREE' amber alert on their altimeters.
Also, the setting of the Altimeter (
to QNH) is part of Boeing's 'Approach Checklist'.
Accordingly, in a B737-NG, if ones fails to set ones Altimeter to QNH (
during descent & approach) there are at least two chances for a crew member to catch this omission, i.e. firstly via the visual 'boxed' amber cue on their altimeter, and secondly via the Approach Checklist.
And lets not forget that the Radio Altimeter would be making aural callouts at various altitudes (
airline specific) and that these should normally be cross-referenced to the Altimeter(s) to confirm a level agreement (
aka. 'airmanship').
The above not withstanding, there is nothing to prohibit the pilots from both having set an identical but incorrect QNH, in which case the above would all function correctly (
aside from the Radio Altimeter being in error) in terms of procedure & matched settings, albeit that the aircraft would not in fact be at the correct altitude.
Nb. this would be a fairly major f**k-up for both pilots to make, but certainly not unheard of.
Given that the approach being flown was, by all accounts, a VOR, there are a number of way that this can be flown in a B737-NG, typically either via the autopilot and / or with reference to the Flight Director(s):
1) Fly it by use of VOR/LOC and Vertical Speed (
with reference to distance versus height, from off of the relevant approach chart), with either the MDA set (
or the closest 100' above) on the MCP, therein prohibiting the aircraft (
if being flown via the autopilot / flight-directors) from descending below MDA (
but which risks the aircraft levelling off, which might not be ideal);
OR activating vertical speed mode but setting the Missed Approach Altitude on the MCP, which then allows the aircraft to maintain a Continuous Descent Approach down to, through, and below the MDA… with all the risks that that entails.
2) Select the relevant VOR approach in the FMC and then fly it via LNAV / VNAV, which assumes that all the correct buttons & options are pressed in the FMC and on the MCP. The typical failures here are the pilots forgetting to set a lower altitude on the MCP to allow the aircraft to descend (
and / or then failing to reset the MCP altitude window for a missed approach) or, more insidiously, during the descent towards the runway, the aircraft switches mode from 'VNAV PATH' to 'VNAV SPD' (
as annunciated on the FMA), which - in VNAV SPD - effectively means the aircraft is descending without following the prescribed vertical path: Such a mode change requires careful monitoring of the FMA by the pilots, and probable action to get the aircraft back on to the VNAV path.
3) Select the relevant VOR approach in the FMC and then press the APP button on the MCP and fly the VOR approach via the IAN mode, this being a mode which presents flight guidance in a manner akin to an ILS, albeit that this is still a VOR approach.
4) Fly the approach entirely 'manually' with reference to raw-data and distance versus height (
from off of the relevant approach chart).
Now unless the VOR involves step down altitudes, or very cold weather, or a very steep approach angle, by far the easiest method of flying a VOR (
in a B737-NG) is via the IAN option (
see 3 above) !
Along with this, its likely(
?) that this B737-NG was also fitted with something known as the 'Vertical Situation Display' (VSD) which presents a profile view to the pilots (
on their Navigation Display / ND) of the decent phase down to the runway, including symbology to show the aircraft position wrt the planned descent profile and also its projected descent path.
So, in order to fly a perfectly serviceable B737-NG into ground / ocean, when following a VOR approach, one would either had to have suffered or instigated a number of systematic and / or SOP failures and / or major loss of situational awareness. Failing those, it must be an atmospheric anomaly (
such as windshear), though one wonders why the wind-shear alerting function (
also fitted in the B737-NG) did not activate, and for which the only course of action is an immediately flown go-around using all available power (
and aggressively applied)!