Non-precision approach in a modern jet

[Nicholas49]

Firstly, I apologise if this has been asked before. I've searched but had no luck.

Having recently flown to a back-of-beyond airport with no ILS, I wanted to ask about non-precision approaches.

In a nutshell, my question is this: when you fly a non-precision approach in a modern aircraft such as the B737NG, how much assistance do you get from the aircraft's computers? Do you still use the airport's VOR for navigation? Since the VOR does not actually guide you to the runway, how do you manage to line up the jet with the runway centreline, especially when weather conditions mean you can't see it?

Also, is it possible for the aircraft to fly an NPA automatically, including the altitude step changes and turns marked on the chart, or must the pilot fly it? manually

I would be very grateful to any patient pro who could provide a succint explanation!

[JAR]

Embraer 195 - if it is the FMS just press approach button and if in VNAV it will fly the whole thing.

VOR/NDB must still be used as back-up and it places you at the same missed approach point but more accurately. Still offset but pointing at the threshold.

[potkettleblack]

The A320 series will do the whole thing fully managed. It essentially uses the GPS backed up by us verifying what it is doing through manually tuning in the required navaid such as an NDB, LOC, DME, VOR etc. As part of the approach briefing we verify the integrity of the database including the platform altitude that we start down on and any step altitudes and the distances between each fix. We are prohibited from amending the database at all as a further safety measure.

The entire procedure can be flown fully managed by the auto pilot only leaving us to disconnect once visual and land. As you rightly point out many non precision approaches are offset so we get to do a little more work to ensure the aircraft is correctly lined up with the runway and that we are stablised and configured correctly in accordance with our company SOP's.

Its a superb system that Airbus have come up with and in my opinion enhances safety to a high degree but like everything its a case of garbage in garbage out so we are very careful to make sure we know what the aircraft is doing and why.

[Rainboe]

B737NG. Standard procedure to use autopilot, but not VNAV. We like to control when to descend and how rapidly to do so, co-ordinating flaps and undercarriage at best times. You always have the aids tuned as backup, and with the track achieved line displayed, you aim to line that up with the final track on the map display so that you are on the exact corrrect track. NPA limits are higher, autolands not possible, so you have to go visual and revert to hand flying to line up on the runway for touchdown. Go arounds would be with autopilot left engaged. In the simulator, you usually also have an engine out just to make it sporty! This makes it far harder because as you change power to correct speed or descent, it induces a strong yawing moment which messes your tracking up if you are not prepared.

[411A]

At our company, it is left entirely up to the handling pilot which autopilot modes (or indeed, no autopilot) are used, for these approaches.
I normally hand fly the entire procedure, including circling, if necessary.
Our First Officers normally do the same.
Type, L1011.

[tom775257]

I fly the A320/1, at this time we are not allowed to carry out a 'managed non-precision approach' at the airline I work for so it is all selected; by this I mean we use raw data to fly the approach not just to monitor. But... the A320 makes this very simple with a mode called Track/FPA (Track / flight path angle). Basically once you are on your QDM (NDB app) or radial in (VOR), you can set the track and the aircraft will keep pretty much the same track all the way down, so you can forget things like the wind backing and slowing as you fly the approach. Flight path angle allows us to continue down a 3 degree slope (or whatever) without having to think about vertical speeds varying with speed. (This is all with reference to Autopilot on, however manually selecting the aircraft's path/speed)

As above, from next sim phase, we will be flying managed NPAs and they essentially become just like an ILS approach.

[Nicholas49]

Thank you to all for your informed replies.

Rainboe, you mentioned that you have the aids tuned as backup, and with the track achieved line displayed, you aim to line that up with the final track on the map display so that you are on the exact corrrect track.

Do I understand correctly therefore that you use the GPS as your primary navigation aid during an NPA, with VOR etc. as a back-up?

Does the 'final track' on the GPS map enable you to line up with the runway without being visual? In this respect, does it perform the same function as the ILS localiser?

Finally, if you are flying a managed NPA, are there electronic versions of approach charts that you load into the system for the autopilot to fly? Do these exist for all non-ILS airports?

Thanks again.

[xetroV]

Whether the non-precision approach is flown in VOR, LOC, or LNAV, using Vertical Speed, flight path angle or VNAV, with or without the autopilot/autothrottle: in all cases the final track adjustments require visual contact with the runway. That's what MDA's (http://www.skybrary.aero/index.php/Minimum_Descent_Altitude/Height) are for.

It is not a very bright idea to leave the NPA track and line up with the runway without being visual (e.g. in case of an NPA with a large offset relatively to the extended centerline), as obstacle clearance is based on the published procedure. Hence the need to monitor the applicable conventional navaids at all times.

GPS is not used as stand-alone navigation source in modern airliners. The FMS position is derived from (AD)IRU, GPS, DME, VOR, and/or LOC measurements, through a complicated algorithm that is based on Kalman filtering and that excludes unreliable signals based on a range of reasonability checks. (At least in theory; there have been several accidents and incidents involving FMS map-shifts, and even GPS is no miracle cure against such occurrences.)