Monday, January 08, 2007

Flying LPV Approaches

NOTE: For a more up-to-date discussion, see Understanding RNAV Approaches.

Continuing my series on the simulating RNAV approaches with the new Garmin 400/500W Trainer software, here are some preliminary results of experiments flying an LPV approach.




I chose the KOAK RNAV 27L approach, which has published LNAV, LNAV/VNAV, and LPV minima.
I initialized my position over TRACY intersection (northeast of the Livermore airport) at 6000 feet, clicked the direct button, and selected KOAK as my destination. Clicking the PROC button, I scrolled down and selected RNAV 27L from the list of choices, and selected SUNOL as the initial approach fix. Since you often get vectored to a fix inside the IAF on approaches into Oakland, I clicked the FLP (flight plan) button, clicked on the small knob on the lower right to enter cursor mode, scrolled to the HEDER waypoint with the big knob, and clicked on the direct button. By the way, loading an approach procedure and then proceeding directly to a fix on that approach effectively activates the approach.

Approaching HEDER, I used the Trainer's throttle slider switch to reduce my airspeed to about 120 knots and the countdown message appears, providing a 10 second warning for the turn to join the intermediate approach course. It's harder to see in this example, but there is a small arc showing the path to fly-by HEDER and become established on the approach. Note that the course sensitivity annunciation is still TERM (terminal mode), meaning a full scale deflection represents a course deviation of 1 nautical mile. Once past HEDER and established, it will be time to descend to cross VODSY at or above 1600 feet. Notice that there is still no clue as to whether the unit will provide LPV, LNAV/VNAV or plain old LNAV sensitivity, so it's best to be prepared for the worst - a non-precision descent to 420 feet and a minimum 5000 foot RVR. Just in case there is going to be vertical descent guidance, it would be best to be at 1600 feet a mile or so before VODSY.

After passing HEDER and joining the approach course, the course sensitivity annunciation changes to LPV, meaning the 530W has the required GPS accuracy to provide vertical guidance and I can use the lowest (LPV) minima shown on the approach chart - 259 feet and a 4000 foot RVR (runway visual range). I just hit the jackpot - the lowest minima - though there won't be a flood of nickels pouring out of the 530W. Descending on the glideslope with the trainer is a bit tricky since you aren't really flying a plane, you're just clicking on an altitude button to descend or climb 100 feet at a time. Descend too quickly when using a 530W that has TAWS and you'll get a terrain warning, even with the Trainer software.





The next installment will investigate flying RNAV departure procedures with the 400/500W Trainer.

5 comments:

John said...

You can use the simulator to fly the lpv approach by pressing the alt button on the simulator autopilot. You should set the autopilot altitude bar to be at the intercept altitude. Don't press the simulator autopilot alt button until you are close to the gs intercept point, or the simulator will cause a climb to intercept the gs.

The Big Pilot said...

Did you try flying the LPV approach using the altitude component of the GSS?

The previous poster had a point but did you try it without using the glideslope? (I've done it using the glideslope freq and it put me on the money. Haven't had time to do it without the Glideslope ILS freq.)

I'm going to try the sim for KGRB to KOSH using RNAC36 approach with COCAP as the IAF.

Sadly, I lack your computer skills so I'll have to post the results but it seems to me that if WAAS is going to be capable, then it should NOT need an ILS beacon.

John said...

John,

You're correct that the ALT button on the Trainer's autopilot panel will track the LPV glideslope. What was trying to illustrate is that if you descend manually using the DN button and you get too high a descent rate, the 530W will display a TAWS alert.


Big Pilot,

I'm confused by your comment. An LPV approach does not require an ILS glideslope transmitter/antenna on the ground to provide descend guidance. I'd be curious to hear about the results of your experiment flying the Trainer from KGRB to KOSH.

John said...

John,

Have you figured out how the cdi sensitivity works when you have lpv, l/vnav, or lnav/+v annunciated. I get varying results, especially if I load and activate an approach using vectors to final. It seems that the cdi sensitivity goes to .3 nm immediately, regardless of the distance to the faf, like 130 nm. I have tried it both with starting on the departure side of the final approach course with susp on and with being generally aligned with the final approach course. On final after the faf, the cdi schedules itself from .3 nm to .06 nm at the runway. This may only be a simulator anomoly.

The Big Pilot said...

Well, I tried my little experiment and found out the simulator will fly a LPV on a glideslope, even if the sim radio is NOT tuned to the glideslope frequency. You have to be sure to configure the altitude on the GPS simulator though.

I just wanted to see if the simulator would do a "no-hands" landing without a Glideslope frequency on a LPV approach.

I re-read my post...I was confused too!! (Should check for typos or not post during "24" ;) ).