MSP Monthly Hint for Jan. 2014: Tired of those ILS Approaches?

This is one of a series of sporadic posts from your MSP Hub Manager telling you more than you want to know about a particular topic. It's been some time since I posted one of these, but if you read below you'll see why. In this case, you're about to learn much more than you wanted to know about a subject you probably have never heard of. Don't worry, it won't be on the quiz.

Those of us who fly the "Heavy Metal" (e.g., large Boeing and Airbus commercial aircraft) know that the descent, approach and landing are the most interesting, and most challenging, parts of a flight. Most of us started out flying "freeware" versions of these aircraft like the ones which are available for downloading on this site. I know I felt pretty good when I learned that I could couple the autopilot to the ILS receiver and have the autopilot fly the aircraft along the localizer and down the glideslope to a point from which I could hand fly a visual landing. I felt even better when I discovered "payware" aircraft (e.g., the Level D 767-300ER) which have a working version of the flight management computer (FMC) which can be programmed to fly the flightplan using naviagtion systems native to the aircraft. I was amazed to learn that, under certain conditions, these aircraft could also perform an "autoland" procedure which would flare the aircraft, touch down, lower the nose, deploy the spoilers, reverse thrust and apply the brakes, leaving it to the pilot to steer the aircraft off the runway to the gate. I also knew that the FMC simulated in these models had some capability for vertical (VNAV) as well as lateral navigation (LNAV).

It wasn't until I bought the PMDG Boeing 738NGX that I found an aircraft that has a real, working VNAV capability. Not only that, it has RNP capability. What is that you ask. Well RNP stands for required navigation performance. It means the aircraft can self-monitor the accuracy of its navigation system and knows the upper bound of the discrepancy between the actual position of the aircraft in three dimensional space and the navigation system's estimate of that position. If that discrepancy is less than a number prescribed for the approach, the navigation system alone (without the ILS) can fly the aircraft to the approach minimums (usually 200 ft. AGL). Check out your favorite large airport's Instrument Approach Procedures (IAPs). You're likely to find something like "RNAV (RNP) Z RWY 18L" for Charlotte/Douglas (KCLT). In fact, both Alaska and Southwest Airlines are now flying RNP approaches on commercial flights where the aircraft are RNP equipped, an appropriate IAP exists and the aircrew has been trained. In time, the FAA expects that circling RNP approaches can be used to drastically shorten approaches and increase airport capacity.

The tutorials which come with the PMDG 738NGX have a flight from London/Heathrow to Amsterdam/Schipol which use LNAV&VNAV to complete the entire flight from liftoff to the touchdown threshold. It's really neat - you can just sit back and watch it happen, as long as you follow the instructions exactly. Translation: don't try this at home.

I have been trying to learn to use this capability on my own SPA flights. I should tell you that none of the capabilities described above is as easy to use as it sounds. There is no big red button on the instrument panel labeled "LAND". In each case there is a substantial learning curve. In fact, the first dozen times you try to use any of these capabilities, even just the "coupled" ILS landing, it isn't likely to work, particularly if you combine it with the demands of communicating with air traffic control, configuring the aircraft for landing, etc. Nonetheless, these are the actual procedures which are used in real commercial aircraft and that alone makes them worth learning, at least for those of us with this particular bug.

So, it is with some pleasure that I report my first successful RNAV (RNP) landing on the above referenced Rwy 18L at Charlotte. Even this accomplishment took multiple tries, including having to fly a missed approach per the IAP and then reassemble the approach on the FMC without the benefit of ATC. While the 738NGX is capable of near autonomous flight from takeoff to landing, it will only work if everything is set up perfectly and if the FMC has sole command of the aircraft. Of course this isn't consistent with any real situation, since one has to be able to repond to ATC instructions and, even with the TAF, it can be tricky to determine the arrival runway in advance. Of course, the FMC has all the flexibility to deal with these contingencies, but that doesn't mean that it's easy to use under the time pressures of a simulated flight. This is what the flight safety gurus mean when they say that cockpit automation can be an aid in routine circumstances, but a hindrance in emergencies. “ You mean I have to fly this aircraft by scanning the gauges and looking out the windows?”

As it turns out, energy management is the key to using the VNAV capability. I learned this by trying to use the system with ATC provided by Radar Contact (or the native FSX ATC). Both of these will give altitude and airspeed clearances which are inconsistent with the FMC energy management logic. There are two consequences. On the one hand ATC will have you descend too early and wind up wasting a lot time and jet fuel. I suspect this is the short route to losing your job in a real airline. In spite of this tendency, the other apparent result is that, by using VNAV to control both the altitude and autothottle from an arbitrary point on the approach, one is very likely to find oneself too high and/or too fast on final. This appears to be the case because the aircraft is not at the altitude/airspeed it would have had if the FMC had been flying the plane from the outset. At any rate, while altitude and airspeed are intrinsically good things, they are hard to get rid of in a hurry and will invariably result in a missed approach or a large pile of burning wreckage on the runway (the former is preferable). Even use of the flight detent on the spoilers and early deployment of the landing gear can be insufficient to bring you down the glideslope at Vref. I had a couple of near successes trying to force a landing when I started too high or too hot.

So, the solution is to kiss ATC goodbye before commencing the final approach. This can be done in Radar Contact by telling them that you are going to fly some specified instrument procedure, or in either ATC system, by cancelling IFR. Then take a look at the FMC and see the altitude and airspeed associated with the first waypoint of the RNAV approach and use LNAV and the autopilot to get to that state. Then engage VNAV and press the "speed intervention" button. That gives you control of the autothrottle which you can use to make the usual step changes in the airspeed and flap settings which you would make on a standard ILS approach. But, instead of the ILS, the autopilot will be using the LNAV and VNAV instructions from the FMC to fly you down the approach. If you set the runway threshold elevation in the MCP elevation window, you will see the green arc on the nav display just touching the runway threshold as you fly down the approach, exactly as you would if you were making a "constant rate of descent" LOC (no G/S) approach using the vertical speed control (Vs) of the autopilot.

In spite of having all the 738NGX Boeing documentation, I haven't yet figured out if it's possible to perform an autoland with an RNAV approach, so you still have to flip off the AP and execute the flare, touchdown and rollout. But you can do it all without ever tuning the ILS receivers.

Addendum added Jan. 19, 2014. Just read this if you are really interested in how this works. By repeatedly reflying the last 10 minutes of this approach I have developed a technique for passing full control of the vertical aspect of the approach from the pilot back to the FMC, that is VNAV control of both the glideslope AND the airspeed. As indicated above, start by using VNAV with speed intervention to hit the first way point on the RNP approach at the altitude and airspeed indicated on the LEGS page of the FMC. It should be possible for the FMC to fly the aircraft all the way down to minimums from this point, and in fact I have gotten it to do that. But to do so you have to realize that the FMC logic computes the commands to be sent to the autothrottle at each point during the final approach based upon energy management calculations which need to "know" the weight and configuration of the aircraft at each point. So you have to make sure these are correct in order to transfer full control to the FMC. The FMC already knows the gross weight of the aircraft, but you have to insure that the aircraft configuration (flaps and gear) are what the FMC has assumed for that point in the approach profile. So, one has to slow the aircraft to an airspeed slightly below the FMC assigned value for the first waypoint and make certain that the flaps and gear are configured as they would be for that airspeed in a normal descent. This assures that the lift and drag of the aircraft will correspond to the values used by the FMC in calculating the aircraft trajectory for the descent. Then, as you pass the first waypoint in the RNP approach, watch for the FMC to issue the throttle retard command to the autothrottle. This shows up in the thrust mode annunciator at the top, left hand side of the PFD. This is the sign that you have intercepted the descent profile, as the FMC begins a descent by retarding the throttle. If anything, you want a slightly lower airspeed and a higher drag configuration than the FMC has assumed in order to ensure that you are not too hot coming down final. This procedure is analogous to intercepting the glideslope from below during an ILS approach. At this point, press the speed intervention button again, returning control of the autothrottle back to the FMC. You will now see the thrust mode annunciator on the PFD change from MCP speed to FMC speed. Thereafter, the FMC will assign a decreasing series of airspeeds to the autothrottle as you extend the gear and deploy the flaps in accordance with the indicated values on the speed tape on the left side of the PFD. This is just what you would do during an ILS approach, but both the lateral (heading) and the vertical (pitch and airspeed) portions of the approach are being controlled by the FMC using the "native" aircraft navigations system. It is obvious that this represents the minimum pilot intervention in any automated approach, as the aircraft's automated systems (AP and FMC) are not empowered to activate the gear or flaps. These have to be hand actuated by the pilot. As you approach minimums you will find yourself at the correct position, attitude and airspeed to disconnect the autopilot and the autothrottle, and flare the aircraft. If you have armed the spoilers and the autobrakes, they will both deploy after the nosewheel touches down and it will remain to you just to reverse thrust and steer the aircraft down the centerline of the runway.

The next step in this little adventure will be to augment this procedure using the heads-up display (HGS) in the PMDG 738NGX. This will make it easier to "nail" the touchdown, which I have been making at night.
<br /><br /><!-- editby --><br /><br /><em>edited by: Westcoast, Jan 19, 2014 - 06:12 PM</em><!-- end editby -->

Thanks Jerry. I've just posted a photo essay illustrating this approach using the heads up display.