Baro VNAV discusion

28.05.2010

Submission:

Discussion between:

Capt. Martin Janiga (CMJ:) – Pilot

and

Ian Whitworth (IW:) procedure designer.

CMJ:

Very high minimums on this Baro VNAV approach, approximately 2000 ft above the runway. What is the critical obstacle?

IW:

It is a long final approach (10nm) because of airspace which restricts the intermediate approach. The critical obstacle is terrain with a height of 1700 ft at about 10 nm.

CMJ:

But at 10 nm I’m at 3200 ft on the glide slope. That is 1500 above the terrain.

IW:

It still penetrates the “W” surface and is critical. When terrain penetrates this surface Pans-Ops says that I have to add a MOC of 75m and so you get the minimum of 2000 ft.

CMJ:

But when I reach the minimum OCH I am at 6 nm from the threshold. That is 4nm Past! the terrain. Are you trying to tell me that the critical terrain determining my OCH is 4 nm behind me when I start the missed approach?

IW:

Yup!

CMJ:

That’s crazy!

CMJ:

Wait a moment! The LNAV approach that doesn’t have any vertical guidance has a lower OCH minimum of 300 ft. How is that possible?

IW:

The LNAV approach has a step-down fix past the terrain.

CMJ:

So put a step-down fix in the Baro VNAV approach.

IW:

Can’t do that. The Baro VNAV approach is an approach with vertical guidance.

CMJ:

Do you mean to tell me that if I switch off my vertical guidance I will get a lower minimum?

IW:

Yup!

CMJ:

That’s crazy!

CMJ:

What about the Baro VNAV approach to the other runway that has a 5nm final? It used to have a minimum of 300 ft and now you have raised the minimum up to 900ft, why?

IW:

We found that with an intermediate altitude of 1700 ft you did not actually intercept the descent slope at 5nm but at 5.2 nm from the threshold.

CMJ:

So?

IW:

That means that the “W” obstacle limitation surface comes into play.

CMJ:

So what? It is the same approach as yesterday.

IW:

Not really, now terrain at 4 nm penetrates this “W” surface and it becomes the critical obstacle.

CMJ:

Why can’t I intercept the descent gradient at 5nm?

IW:

Because of the minimum in the intermediate segment

CMJ:

So yesterday when I flew the approach and nominally intercepted the descent slope at 5nm I had a lower minimum that today when I intercept the descent slope at 5.2nm. Are you telling me that yesterday this terrain at 4nm was not a problem and today it is, just because of where I start my descent?

IW:

Yup!

CMJ:

That’s crazy!


.

Answer or Commentary:

Simple solution:

Make a note on the chart that SBAS Avionics with Angular Vertical Guidance are not approved to provide Baro-VNAV guidance.

That assures that you do not have to assess the "W" surface and can use the Baro FAS only.

Beat Zimmermann - Air Navigation Institute - Switzerland but presently in Slovakia

.