We’re The Fugawi

A Joe Walsh lyric from the early 70’s says “You can’t know where you’re going if you don’t where you are”.  No longer true.

Modern airliners use a Flight Management Computer (FMC) to program a Flight Management System (FMS) which gets position information from Global Positioning System (GPS) and backs it up with a redundant Inertial Navigation System. This is a gross over-simplification but most of this is transparent to even the pilots so it doesn’t matter anyway.

We get a paper flight plan that is set up like a spread sheet. It shows all of the points along the way. We call them waypoints for some reason. Nobody knows.

So long as the paper flight plan agrees with the route stored in the FMC and the clearance that the Air Traffic Controller (ATC) reads to us over the radio or sends to us by Instant Message over the ACARS system (I can’t remember what ACARS stands for) then we execute the plan. This all happens while we are parked at the gate. It’s a lot less complicated than it sounds but it is really easy to screw up.

Position is updated again near the end of the runway just before takeoff in case a few feet of error was induced during the taxi out.

After takeoff we can navigate several different ways. The FMS system can be engaged with the autopilot and will fly to each programmed waypoint.

Pilots generally keep track of about where they are. There should always be an inverse relationship between altitude and positional “assuredness”. Meaning, the lower you are the more it matters where you are. Above ten thousand feet there just isn’t much terrain in the Continental United States to worry about. Navigation still matters but it usually isn’t critical in the precision sense, but getting lost will make your ass sure red. (This is probably something Bob said, I got it from somewhere)

On really long flights with several pages of waypoints and several turns along the way Air Traffic Control will often approve shortcuts that straighten out the route and shave off a few minutes of flying time.  This will take us several miles off of the planned route. Additional deviations to avoid severe weather can easily add up to more than a hundred miles from the original flight plan.

All this sums up to: Even if we do know what lake or even what State we are over chances are we don’t really care.  Just so long as our destination is at the end of the Magenta Line on the big moving map display and the fuel numbers add up right everything will be just fine. Unless of course, the Russians decide to shoot us down. But that is a different story.

Happy Landings.


4 Responses to “We’re The Fugawi”

  1. Since I used to work for a company that made Inertial Navigation Systems, I thought I could help explain :


    The equipment, and hence the aircraft, knows where it is at all times.
    It knows this because it knows where it isn’t.
    By subtracting where it is from where it isn’t (or where it isn’t from where it is, depending on which is greater), it obtains a difference or deviation.
    The inertial reference system uses deviations to generate corrective commands to fly the aircraft from a position where it is to a position where it isn’t. The aircraft arrives at the position where it wasn’t; consequently, the position where
    it was, is now the position where it isn’t. In the event that the position where it is now, is not the same as the position where it originally wasn’t, the system will acquire a variation. (Variations are caused by external factors, and discussion of these factors is beyond the scope of this simple explanation).
    The variation is the difference between where the aircraft is and where the aircraft wasn’t. If the variation is considered to be a significant factor, it too may be corrected by the IRS.
    The aircraft must now know where it was.
    The “Thought Process” of the equipment is as follows:
    because a variation has modified some of the navigational information which the aircraft acquired, it is not sure where it is.
    However, it is sure where it isn’t and knows where it was. It now subtracts where it should be from where it wasn’t (or vice-versa) and by
    differentiating this from the algebraic difference between where it shouldn’t be and where it was, it is able to obtain the difference between its deviation and its variation; this difference being called error.


  2. HI Luckyjet having just returned from a flying vacation – SFO- LA -NO -DEN- SFO I have a couple of questions for you.

    1 – who decides the layout of the plane? rows, number of seats in rows, shoulder width, leg width? Persuming is’t the powers that think they are do they ever spend time flying in their creations?

    2 – weather was seriously fowl on approach into SFO. It felt as it the clouds went on for miles as we decended. Does that freak pilots out as much as it does passenger?

  3. luckyjet1 Says:

    Regarding the cabin layout, seat “pitch” etc… The individual airlines specify pretty much everything including the galley layout, seat size, lavatory location, and fabric choices. The FAA has some clear rules regarding “Emergency Exit” seat rows, floor lighting, and all those silly signs and announcements.

    How high up the clouds go doesn’t really bother pilots so long as the clouds aren’t part of a storm. It is pretty easy to tell the difference, even without radar. There might be a little ice up there but it isn’t a problem for a jet.

    How low the clouds go does make a difference to us but airliners and airline pilots are generally equipped and qualified to fly down to about as high as you can throw a rock before we see the runway and still land safely.

    Part of the reason it doesn’t bother us is that we know how high we are. When sitting in the back there is just a continued descent into nothing.

    Usually at 18,000 feet the landing lights are turned on. They are really bright. Most airlines ring a cabin bell at 10,000 feet. This should be around 10 minutes prior to landing. Maybe 15.

    Generally you can tell when the landing gear is extended and the flaps are fully extended. This will usually be about three minutes before your happy landing.

    I’ll bet you meant seriously foul …. else I’ll have to tell you all about how we avoid hitting the birds.

  4. um, yeah. Thanks for the info.

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