Six Feet From Impact: How to Ruin a Baro-VNAV Approach

During a go-around after a Baro-VNAV approach, Airhub Airlines flight 4311 missed the ground by 6 feet. At this point, the pilots were completely unaware of how close they came to crashing the aircraft. How did this happen, and what can we learn from this serious incident?

We are going to cover some important elements that we need to understand first, to get a clear picture of what happened. However, if you already understand the ins and outs of Baro-VNAV and its main risk, just skip straight to the section covering the incident flight!

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What is a Baro-VNAV Approach?

A Baro-VNAV approach is a type of Required Navigation Performance (RNP) Approach that relies on barometric altitude for the vertical guidance. Baro stands for barometric, VNAV stands for Vertical Navigation.

What this means is that, when you fly a Baro-VNAV apporach, you will get lateral guidance using GNSS, but the glideslope you’ll be flying is based on your altimeter, not GNSS.

This is a substantial difference compared to other RNP approaches like an LPV approach, where both horizontal and vertical guidance are both achieved by using GNSS.

To wrap your head around the terminologies, have a look at this table. The column on the left has the approaches that are affected by barometric altitude and therefore your altimeter setting. The ones on the right are not affected by barometric altitude.

As you can see, there are a lot of approaches affected by altimeter settings! The tricky part of a Baro-VNAV approach is that it looks exactly like a precision approach from the way your PFD will present the glideslope information. However, you still have to rely on the published altitude vs distance checks to verify you’re on the correct glidepath.

The Risk of Baro-VNAV Approaches

The biggest risk of Baro-VNAV approaches is that the wrong altimeter setting can completely distort your perception of where the glideslide is, versus where it actually is.

Because the entire system is referencing your barometric altitude, it relies entirely on pilots setting the correct QNH. This means that if you set the altimeter on the wrong QNH by 1 Hpa, your altitude will differ by 28ft* from the required altitude along the glidepath.

If your pressure setting is higher than the actual QNH, you will be flying below the desired glidepath. If your pressure setting is lower than the actual QNH, you will be flying above the desired glidepath.

This is the case for International Standard Atmosphere (ISA) conditions. However, when it’s colder than ISA conditions, it’s less than 28 ft. At higher temperatures, it’s more than 28 ft per HPa!

The worst thing about this, is that even the distance versus altitude checks are based on the same barometric reference! So by reading out the checks with a wrong QNH setting, you will still not detect that something is wrong!

The Incident Flight

On May 23rd 2022, an Airhub Airlines A320 was on its way from Stockholm Arlanda to Paris-Charles de Gaulle.

Knowing that the ILS was not in service that day, the crew briefed for an LNAV/VNAV approach into Paris, as they did not have the required equipment for an LPV approach.

Enroute Phase

The crew listened to the ATIS when they were flying above The Netherlands. This is what it reported:

As you might have guessed, the last item is going to be the main focus here. When speaking to the Paris Controller, the pilots received a clearence to descend to 6000 ft on QNH 1011 (which is incorrect). Note that this QNH is different by 10 Hpa, which according to what we discussed earlier, comes down to a difference of -280 ft!

A few minutes later, the same controller gave the instruction to descend to 5000 ft and cleared the pilots for RNP approach runway 27R and then repeated the same incorrect QNH.

The controller interestingly also gave the wrong QNH to an inbound Easyjet flight, but the QNH that was repeated back was the correct one from the ATIS. Unfortunately, the pilots did not pick up on this.

The Baro-VNAV Approach Phase

The crew completed their approach and landing checks, and they were fully setup on the 27R RNP Approach track. The entire approach was mostly flown in IMC, with poor visibility due to passing CB’s.

They were flying with an indicated altitude of 4900 ft, but were actually only flying at 4620 ft. The radio altimeter became live at 2500 ft and despite the difference in actual and indicated altitude, they were ‘established’ from their perspective.

Their decision height was 752 ft according to the plate, based on LNAV/VNAV minima for CAT C. The crew had a procedure to add 50 ft to the LNAV/VNAV minima, resulting in a Derived Decision Altitude (DDA) of 802 ft.

The airport is equipped with a system called ‘MSAW’, which alerts ATC when aircraft descend below a certain threshold. When they were flying through an altitude of 891 ft (remember, an actual altitude of 611 ft, which is 211 ft above aerodrome level), they were still over 1.5 nm away from the threshold. This caused MSAW to generate an alert, which caused ATC to ask:

“I just had a ground proximity alert, are you okay? Do you see the runway?”

The crew missed this transmission and did not reply, likely due to workload.

The Go-Around Phase

During the ATC transmission at 52 ft AGL and an indicated altitude of 802 ft (their DDA), the captain disconnected the autopilot, and applied pitch to initiate the go-around. The report indicates none of the pilots were ever visual with the runway or the ground.

Three seconds later after initiating, the aircraft reached its lowest point during the go-around of 6 ft above the ground, according to radio altimeter data.

Keep in mind that from their perspective at this point in time, nothing about the approach they just flew was out of the ordinary. They called the go-around, got vectored to fly heading 360. They then set up again for the same approach to runway 27R.

They were cleared to 5000 ft on (the correct) QNH of 1001, but they read back the wrong QNH of 1011, which unfortunately was not corrected by ATC…

Before you think that the entire thing is going to repeat itself, don’t worry. This time, because of a slight weather improvement, they were actually visual with the runway before the decision point, which meant that they spotted the fact they were very shallow (again) and managed to correct for it. This resulted in a safe landing.

Learning Points from this Baro-VNAV Incident

So what are the main take-aways? Well, most of all:

1) Check the QNH setting. Gross error check it whenever you get your first clearance. If you’ve listened to ATIS before that point, compare it to that and see if it makes sense. Of course it could be slightly different, but probably not by 10 HPa.

2) Operators should establish SOP’s to make sure that setting any QNH before an RNP approach is cross-referenced with independent data. Whether that’s GPS altitude or radio altitude.

3) Listen out to other transmissions. In this case, the crew didn’t hear some of the transmissions to other aircraft like the easyJet flight we mentioned. If they did, it could have made the difference.

Conclusion

Please keep in mind that this is a simplified breakdown of the incident. If you’d like to dive deeper and learn more about the details during this event, please go to the official preliminary report, which you can find here.