Thursday, February 12, 2009

JungleBus Systems Post: Flight Management System

Jeeze. It's been a while since I've posted. If I still have any readers left - sorry! I turn around for one second, life happens, and it's suddenly nearly a month since I've written for the blog. I actually have weekends off this month - probably a one-time fluke, which I'll write about in another post - so I've been spending my days off with Dawn. That's certainly a good thing from my perspective, but it does mean less time for writing.

I have a good story that happened to me about a month ago, but understanding it requires some knowledge about the JungleBus' Flight Management System (FMS). That's as good of an excuse as any to revive the JungleBus Systems series, along with the fact that a reader specifically requested a post on the FMS a few months ago. I'll write about the FMS in this post, and tell you the related story in my next.

One note: the FMS is one system where I think it's prudent to keep some details vague. Information on how various aircraft systems work isn't generally security-sensitive - someone with nefarious intentions probably isn't too interested in the purpose of Electric Hydraulic Pump 3A! - but the FMS is an exception. I have no interest in helping some Mohommed Atta wannabe navigate their way to Washington DC, so I'll confine myself to theoreticals rather than a detailed walk through of the FMS' functions.

The Flight Management System in the JungleBus is manufactured by Honeywell and is an integral part of their Primus Epic integrated aviation system. To light plane pilots, the Primus Epic is most analogous to the Garmin G1000 system. It covers a great many of the features in the JungleBus' cockpit: the flat panel Primary Flight Displays, Multifunction Displays, Engine Indication & Crew Awareness System (EICAS) Display, the trackpad-like cursor control devices, the autothrottle, flight director/autopilot, FMS units, communication radios, ground proximity warning system (EGPWS), and Multifunction Control Display Units (MCDUs). Because all these components are so interrelated, it's hard to delineate exactly where the FMS ends and another component begins.

Many of the Primus Epic's components are visible in the above photograph - but not the FMSes. They reside in the belly of the airplane, in several Modular Avionics Units (MAUs). The identical boxes on the forward portion of the center pedestal are commonly referred to as FMSes, but they're technically MCDUs. The MCDU is the pilots' only interface with the FMS. On some airplanes, that's all that the MCDU does, so the terms MCDU and FMS became roughly interchangeable. The Q400 is that way; you can actually turn the MCDUs/FMSes off in flight if you want to, and all you'll lose is GPS navigation. On the JungleBus, though, the MCDUs also handle a number of non-FMS functions such as communication and navigation radios, ACARS (Aircraft Communication Addressing & Reporting System), and engine thrust setting selection. The FMS itself has a number of non-navigation functions that are separate on less integrated airplanes. I'm guessing that we use the MCDUs more than any other single peice of equipment on the airplane - including the control yokes! There is no way to turn off the MCDU on the JungleBus short of pulling circuit breakers.

The most basic feature of the Flight Management System is navigation. It's easy to think of the FMS as an overbuilt GPS unit, but GPS is actually only one of the signals that the FMS considers in determining aircraft position. The Inertial Reference System (IRS) also provides input. Surprisingly, the system also automatically tunes and triangulates good old fashioned VORs and DMEs to help determine position. From all these inputs, the system not only determines aircraft position but also calculates its own margin of error, a number known as ANP (actual navigation performance). Most of the time, ANP is less than .1 nautical mile. If ANP exceeds certain parameters for varous phases of flight, or if the two FMSes disagree with each other, the pilots get a warning that navigational performance is degraded. If GPS signals are lost for whatever reason, the system can still do a reasonably accurate job of determining aircraft position using only IRS or VOR/DME.

Navigation is accomplished by entering waypoints into the FMS' flight plan. It uses an internal worldwide database of airports, navaids, and airways that's updated every 28 days. It's relatively easy to make a mistake while entering a flight plan, and this has led to more than one accident in the past. For this reason, the FMS will not actually use any changes to the flight plan for navigation until you go through the second step of activating it. It's standard operating procedure for both pilots to thoroughly review the flight plan before any changes are activated. The Primus Epic makes this easier by displaying the proposed route on the Multi-Function Display as a dashed line. The complete cleared route, including destination and alternate airports, is always entered, reviewed, and activated before flight.

Besides entering the flight plan, the pilots will also initialize the performance function of the FMS before flight. This involves entering speed profiles, cruising altitude, average wind & temperature at cruise altitude, zero fuel weight, and reserve & holding fuel. The FMS automatically gets fuel on board numbers, and calculates time and fuel required to each waypoint. This is automatically updated while enroute, since the FMS calculates current winds aloft and incorporates this into its calculations. The FMS' fuel calculations can easily be checked against the printed flight plan prepared by dispatch. The FMS also has a handy "What-If" performance function where you can check the effect that changing altitude or speed will have on time and fuel required to your destination.

Assuming that the performance function has been initialized, the JungleBus' FMS is capable of not only lateral but also vertical navigation. Wheras the Q400's VNAV was only usable for descents, this one can be used for both climbs and descents. This is particularly handly for RNAV departures and arrivals, where there may be multiple crossing altitude and airspeed restrictions in a fairly short period of time. As long as these restrictions are properly loaded into the flight plan, compliance is as easy as setting the flight guidance panel's altitude selector to the final cleared altitude, coupling VNAV as the vertical mode, and setting the speed selector to FMS. Of course this can lead to complacency, and more than one pilot has busted their clearance by entering restrictions into the FMS improperly or asking the airplane to do something it physically cannot do. You really need to keep a close eye on the magic to make sure it's doing what you want it to do.

Like the Q400, the JungleBus' FMS is approach approved, with the VNAV usable for approach. We can fly VOR, GPS, or NDB approaches using the FMS, with a vertical path on nearly every approach. Interestingly enough, we don't even have an ADF receiver in the airplane to receive NDBs, so we can only fly those approaches if they have GPS overlay. Unlike the newest general aviation boxes, we cannot use LPV minimums - we use the LNAV/VNAV minimums. Given how many major airports with ILSes that we fly to, I don't shoot FMS approaches nearly as much as I did at Horizon.

One feature that the JungleBus FMS handles that's critical to my next post is takeoff and landing speed bugs. These are displayed on both pilots' Primary Flight Displays and are called off by the pilot not flying. The First Officer normally enters the takeoff speeds in the FMS during his preflight flow. He uses the same menu to set the takeoff flap setting, which the takeoff configuration warning system uses to verify that the flaps have been properly set when the thrust levers come up for takeoff. During the same flow, the FO uses another menu to set the takeoff thrust setting; this is technically not part of the FMS, but another MCDU function. One could operate the JungleBus without FMS navigation easily enough - there are still airliners like the DC9 that do it every day - but without MCDUs you'd be in a very unusual situation indeed. This is an important distinction for my next post.

Overall, I think the JungleBus' FMS is pretty well-designed. It's fairly easy to use once you get used to it; the software seems to have been designed by pilots rather than engineers. There are a nice few features that the UNS-1E in the Q400 has that this one doesn't, but the Q400's FMS isn't nearly as well integrated with the rest of the airplane. My chief complaint with the Honeywell unit is that it's awfully slow sometimes; we often joke that they used recycled 286 processors. The VNAV is also rather glitchy; you just have to keep a close eye on it and sometimes use other autopilot modes to ensure smooth transitions.

One final comment is that the FMS that's in the JungleBus is a far cry from the FMS that was in the airplane when I went through initial training. It's the same hardware, to be sure, but the software has gone through several major revisions since then. Whole menus have changed in some cases. The funny thing is that the simulator was one revision behind the airplane even when I went through initial training, which made for a few surprises during IOE. The sim's revision still hasn't changed since then, so when I go back for recurrent it feels like I'm being tested on my historical knowledge of JungleBus software loads!


Ross said...


Thanks for the rundown on the FMS.

Keep posting technical articles as I find them very interesting


Aviatrix said...

The old post doesn't seem like nearly a month old because the topic remains valid. Feel free to leave this one up for a while, it bears more than one reading.

Anonymous said...

You joke around but I have read actually that the MCDU in the Airbus 32x series uses (or at least used) the 286. Also that the 777 uses 386 or 486 processors, and that the shuttle flew on 8086 processors before being upgraded to pentiums. I can't seem to verify any of this other than anecdotal comments in forums so I will disclaim on that point. I would imagine that newer aircraft and newer deliveries of aircraft are probably being upgraded to faster processor designs.

But the point is that these old processor designs actually are quite useful in applications like these because their reliability has been proven with years of track record. Also, "simpler" circuits like the 286 have been made into radiation-hardened circuit designs that can be used on spacecraft, satellites, etc. It would be too costly and large to manufacture the more recent more complicated processor designs in radiation-hardened forms.


Anonymous said...

Well done, Captain!!

Matt said...

Awaiting your thoughts on the MegaWhacker accident near BUF.. How well did it handle ice in your experiences?

Anonymous said...

I like how you say that "VNAV is also rather glitchy"... You would think that all the certification that it went through would have taken care of the "glitchiness"!

Fred said...

V interesting, Sam.

I have to say that since I've started flying these magnificent sims (PMDG 747-400, Level-D 767, PMDG 737), the modelling they've done of the FMS/FMC, etc. is beyond belief. Incredibly life-like.

I'll get some questions together for VNAV operations ... especially for RNAV/GPS approaches and how you figure descent rates. I've heard of the CDAP, do you use those?

zb said...

Norman/Anon@7:13 seems to be very correct about seemingly ancient processors in safety-sensitive applications. While there may still be a slight chance of hidden bugs in modern processors, stuff up to the 486 or pentium era is very well known. Add the fact that these processors are less complex, thus easier to understand and easier to program when secure programming and compiling mechanism are used.

Sometimes, three of these processors are used at the same time, running software that was programmed by independent teams with different programming environments, and commands are only issued when at least two out of three processors 'agree'. Another strategy might be to use two different architectures, say, a pentium and a PowerPC, and have them agree before doing something that's critical to safety.

It's really very interesting to dig into these safety strategies, and I'm happy that my new employer uses them wisely. A lot of stuff yet to learn ;-)

Aluwings said...

re: "The sim's revision still hasn't changed since then, so when I go back for recurrent it feels like I'm being tested on my historical knowledge of JungleBus software loads!"

It's always a challenge to the training departments to keep up with changes on the line - especially when management doesn't recognize the need and apportion the resources.

Unhappily, significant portions of our training (both intial and recurrent) consists of "negative training" that we have to overcome, rather than learn from.

Rick said...

What is the basic difference between an arrival chart and an instrument approach procedure chart?

Hans said...

If you look next time you're flying, your ANP in the JungleBus is usually somewhere around 0.03 give or take. That's a circle 180 feet in diameter. When you consider that the wingspan itself is 85'4" and the length of the plane is 103'11" the plane itself takes up quite a bit of that circle. Not half bad indeed considering its all math, clocks and a little bit of magic to rule out that the airplane is anywhere else outside of that circle.