Snow apparently also has an effect on GS reliability near minima.
Dave_Phillips wrote:
Cork had options, which only makes it more of a waste. However they flew below minima and, when they finally made the correct decision, ran out of luck and skill.
I don’t think the Cork accident is really relevant. They lost control going around: the accuracy of the ILS was not a factor. The same thing could have happened at normal DH on the first approach, with the same result.
Assuming that we’re flying a light piston aircraft, in that “out of options” scenario, the landing performance is unlikely to be limiting. So why follow the GS below DH? Why not just follow the LOC and accept you may land a few hundred metres past the TDZ?
bookworm wrote:
Why not just follow the LOC and accept you may land a few hundred metres past the TDZ?
The issue that those of us who have operated a good deal at low vis have identified is not the risk of landing long, but of the tendency to go below the slope and hit the ground short.
Timothy wrote:
This is the same question as “how would I know I was on the FAT of an overlay NDB approach if I had no ADF?” or “How do I know I can rely on an SBAS VNAV slope when it was defined for BaroVNAV?” The answer is that no-one in officialdom will agree that you can, but you know perfectly well you can.
In the US, the SBAS VNAV slope is approved in lieu of the Baro VNAV slope. The FAA approved it based on meeting certain criteria for the FSD (Full Scale Deflection) values. These are defined in RTCA DO 229 any version. The SBAS slope does not have the temperature limitations as it is relatively fixed in space. In effect, once the slope is outside of the point where the FSD reaches 150 meters, it becomes fixed at 150 meters. Inside this the FSD is angular and FSD is +/- 0.25 times the glide path angle until it reaches a point where FSD is +/- 45 meters and stays fixed at +/- 45 meters from that point until intercepting the GPIP (Glidepath Intercept Point which is the runway if the runway is level). For a three degree slope the 45 meter FSD point is approximately 1.7 NM to the threshold and the 150 meter FSD point is approximately 6 NM from the threshold.
We now have very few approaches that are LNAV/VNAV but that do not also have LPV minimums, so it is mostly a moot point.
NCYankee wrote:
We now have very few approaches that are LNAV/VNAV but that do not also have LPV minimums, so it is mostly a moot point.
Lucky you. We have several LNAV/VNAV approaches in the UK (counting the Channel Islands as UK for the sake of this argument) where the SBAS VNAV component has been switched off by Jepp at the insistence of our CAA, who have taken the, on the face of it, ridiculous decision that the risk of flying with no glideslope is less than the risk of flying with a glideslope which has been surveyed to BaroVNAV standards but not SBAS VNAV.
They haven’t made Garmin switch off LNAV+V glideslopes, which haven’t been surveyed at all, mind you.
It is this mindless “There is a tiny Type 1 risk, so let’s go with that and forget the big Type 2 risk” argument that makes me go wibble, wibble, wibble in these discussions ;-)
bookworm wrote:
They “work” yes, but the integrity is a bit scary. It promises to tell you within 6.2 seconds (TTA) if it’s lying to you by more than 50 m (165 ft) (VAL) about your vertical position. That’s not much of a promise when you’re below 200 ft. :)
Almost True,
But the likelihood of the VPL exceeding the VAL is very remote as this is the 5 sigma possibility that the true position will be outside of the actual position 99.99999%. Note that the 50 meter VAL value only applies to approaches that have a DH of 250 feet or greater, to get a DA below this, the VAL needs to be tighter at 35 meters or 115 feet. The typical VFOM (Vertical Figure of Merit) is a 95% confidence value and is more like +/- 40 feet in the worst case. Also, since the DA altitude is based on the baro altimeter, in the worst case scenario, this has the effect of moving the point at which the aircraft reaches the DA backwards along a lowered slope, but well inside the tolerance required for Baro VNAV or LPV. Below that point, one is supposed to be visual if they continue anyway, so the VPL value should be irrelevant past the DA.
NCYankee wrote:
Also, since the DA altitude is based on the baro altimeter, in the worst case scenario, this has the effect of moving the point at which the aircraft reaches the DA backwards along a lowered slope, but well inside the tolerance required for Baro VNAV or LPV. Below that point, one is supposed to be visual if they continue anyway, so the VPL value should be irrelevant past the DA.
Yeah but the discussion is about what happens if you do follow the needles below the DA.
Timothy wrote:
The issue that those of us who have operated a good deal at low vis have identified is not the risk of landing long, but of the tendency to go below the slope and hit the ground short.
What evidence is there for that? If you are on slope fully configured it seems unlikely unless you start pitching down.
I think the problem is that pitching down is what people do if they start using other cues for vertical profile instead of the GS once they become visual in low vis conditions.
Fuji_Abound wrote:
What evidence is there for that? If you are on slope fully configured it seems unlikely unless you start pitching down.
Only long experience of watching people do it. Actually, it doesn’t even need particularly bad conditions, it just seems to happen at 200’ when people look out.
And yes, it is because they pitch down.
I don’t have any theoretical basis, just observation.
