Tuesday, May 01, 2012

Out on a Limb, G1000 Style

As the FAA continues the inexorable march toward eliminating most ground-based radio navigation aids it would seem pilots who learned radio navigation with NDBs and VORs have two choices: Continue to deny the ever-growing dominance of Area Navigation (RNAV and GPS) or jump in and start swimming with the rest of us. While there's a lot to like about RNAV, the complex implementation of these systems and procedures makes for a steep learning curve. And the learning never stops because RNAV rough edges always seem to crop up when you least expect them. Here's a situation that may leave even a seasoned G1000 jockey asking "Why's it doing that?"

Late Missed Approach

Getting a late start on the missed approach is risky business, especially on a non-precision affair when the missed approach point is some distance from the runway threshold. You might think this could never happen to you, but here are just a couple of scenarios where you could find yourself executing the missed approach late in the game.

Scenario #1: You break out of the clouds well before the missed approach point (MAP), and continue descending. Soon you realize you're behind the aircraft and you won't be able to land in the touchdown zone. So you decide to climb back to the circling MDA and circle to land, but as you climb, you re-enter the clouds and must execute the missed approach.

Scenario #2: You break out on an approach and have the required visibility to continue for a straight-in landing. Only on short final do you realize there's a disabled aircraft on the runway. If you prefer, substitute a herd of elk for a disabled aircraft. VoilĂ ! Late execution of the missed approach.

You Want Me to Do What?

When starting the missed approach beyond the MAP and below the minimum descent altitude, the guidance given by a WAAS GPS can be confusing unless you know what to expect. I first noticed this while supervising a pilot flying a practice RNAV RWY 32 into Cloverdale. This brings up a third possible scenario: Flying a practice approach with the plan to do a touch and go landing and then fly the published missed approach. At the MDA, I asked the pilot to look up and continue visually so as to continue a stabilized approach for the touch-and-go. Note the 881 foot altitude nestled between TAKUY and NATIC? More on that later.

At the MAP, below MDA, on glide angle to runway.
During climb out after the touch and go, the pilot pressed the SUSP softkey on the G1000 Primary Flight Display (PDF) and we both expected instructions to execute a climbing left turn to 6000 feet. Instead, the G1000 wanted us to climb straight ahead to 881 feet, significantly below the circling MDA.

Past the runway, waay past the MAP, G1000 says to climb to 881 feet.
The pilot donned his view-limiting device and dutifully complied with the machine's instructions while I monitored the situation (squirming in my seat, I might add). After reaching 881 feet, the G1000 finally told us to start the climbing left turn to 6000 feet. Ironically, this put us right over some of the highest terrain in the airport vicinity at an altitude that was just barely sufficient for VFR.

Good climb rate to 6000 feet, but still a lot of yellow and red showing ...

Why Would a GPS Do That?

The first important lesson has to do with why a G1000 (or presumably any WAAS GPS) would exhibit this behavior when operating below the MDA, near or past the MAP. Instrument approaches are coded using ARINC-424 by the company who creates and supplies the database (Jeppesen, in this case). The GPS in your panel executes the instructions contained in the database when you execute an instrument procedure. When I learned this I had a deeper appreciation of the complexity of creating GPS databases and found myself less likely to complain about database subscription prices.

While I don't claim to possess detailed knowledge of ARINC-424, a brief explanation from the folks at the FAA's Instrument Approach Procedures division did shed some light. When you pass the final approach fix (FAF), descend well below the MDA, and then press the SUSP softkey, a CA leg (climb-to-altitude) becomes active as a sort of glue between the final approach segment and the missed approach segment. The altitude for the CA leg is computed at the MAP as if you were on the final approach segment, descending from 2500 feet at the FAF at an angle of 3.09 degrees. Since the MAP is 4.9 miles from the FAF, performing a little math shows that you would descend to about 880 feet at the MAP. This computed altitude is even shown in the flight plan, between the MAP and the missed approach holding waypoint. Some pilots mistakenly confuse this altitude with the MDA when, in fact, it is considerably lower. Don't be one of those pilots!

Keep in mind that whenever you choose to operate below the MDA, you are required to have one or more of the visual references described in 14 CFR 91.175.

If you descend below the computed altitude of 881 feet and press SUSP, the CA leg becomes active and the GPS will first instruct you to climb straight ahead to 881 feet. After reaching 881 feet, it will then tell you to start the climbing left turn to NATIC. If you remain at 1440 feet all the way to the MAP and then press SUSP, the GPS will skip the CA leg and immediately instruct you to start the climbing left turn to 6000 feet and proceed to NATIC. Remember that a computed altitude can be displayed in the flight plan and should not be confused with the MDA.

Keep it Straight and Simple

Which brings us to the second lesson: Anytime you find yourself operating below the MDA, especially in rapidly changing weather conditions, you'd be wise to have an escape strategy of your own and not just blindly do what your GPS tells you to do. In the approach cited above, it may be safer to perform climbing turns over the airport until you  reach the MDA, then press SUSP and follow the GPS's instructions. Lastly, remember to fly the procedure as published and that any computed altitudes displayed in the G1000's flight plan should not be confused with the MDA. You may be saying to yourself that this sort of thing could never happen to you, but never say "never." Someday you could find you and your G1000 out on this very limb.


David Cheung said...

Very interesting situation. What would happen if I pressed SUSP and ignored the CA by executing the missed approach as published? Will the G1000 know to skip the CA waypoint and go straight to the hold (or next segment)?

John Ewing said...


You could just climb, turn left, and after passing 881' the G1000 would get in sync with you. The problem is that even though you'd be following the magenta line, you'd not have any guarantee of obstruction or terrain clearance since you started the missed approach past the MAP and below the MDA.

In the flatlands you might be okay. In mountainous terrain, you could be in a world of hurt unless you first navigated back to the MAP.

terps guy said...

If you miss well below and beyond MDA, use the heading mode to properly enter the DF leg. Keep in mind you have an obligation to max out your climb rate to play vertical catch up at the same time.

John Ewing said...

terps guy,

I'm not a TERPs expert, but I don't believe the facts support all of your assertions.

Simply using the heading mode to enter the Direct-to-Fix leg when you are past the MAP can be disastrous on many approaches in mountainous areas. Your best bet may very well be to backtrack to the MAP and then get out of Dodge.

By all means climb as quickly as possible if you need to miss the approach beyond the MAP and/or below the MDA/DA, but it seems pretty clear that with regard to terrain and obstruction clearance you're on your own. TERPs do not provide any objective criteria under these conditions, which is why you are supposed to one or more of the visual references described 14 CFR 91.175.