GPS Drawing [Gallery] [Maps] [Info]

BNN Bovingdon
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It was the GPS from a holding pattern on a commercial airline flight that began my interest in GPS drawing. They are composed of a number of air traffic conditions I find them quite elegant. These GPS tracks are of a beautiful holding pattern near London. I wanted to know more about it so I asked Capt. W. A. Diego, an international airline pilot who replied:

“My very educated guess is that the fix is BNN (Bovingdon) which is a VOR/DME (VHF Omnidirectional Range/Distance Measuring Equipment) navigational facility, frequency of 113.5 Mhz.  The holding pattern is defined on London (Heathrow) Bovingdon one arrival page 10-2A as depicted (Hold Northwest BNN 299 degree radial right turns).  ICAO (International Civil Aviation Organization) further defines the pattern as a leg length inbound to the fix of 1 1/2 minutes above 14,000 feet (max indicated airspeed 240 knots), or 1 minute at or below 14,000 feet (max indicated airspeed KIAS 200).  London control will also assign you an altitude to hold (for separation) and an "EFC" (expected further clearance time) so that you can manage/plan fuel and also to be used in case of communication failure.”



I checked the coordinates and the GPS tracks are indeed from BNN. I then asked:

When holding below 14,000 feet you say "max indicated airspeed KIAS 200"...
How accurate are the airspeed indicators?
How reliable are they and what's your backup?
How many airspeed gauges are there in the cockpit?

He replied:

"Regarding your question on KIAS (knots indicated airspeed)...

Yes, KIAS gages are now very accurate.  In the past there used to be both KIAS and CAS (calibrated airspeed).  KIAS is what you read on the gage, and CAS was KIAS corrected for gage error, position error, mach error, etc.  Usually there was only a 1-2 knot difference, but occasionally 5-7.  All the new equipment has super sensitive, computer generated, extremely accurate KIAS already corrected for everything.  What you see is what you get.  Most cockpits now have three KIAS gages (2 large and one small) each with entirely different inputs.

But, as to how many airspeed indications there are in the cockpit, it is more complicated.  There are two or more of each of the following Speed Indicators in the cockpit now-a-days.

1. KIAS...actually a pressure indication telling you how fast the airplane thinks it's going.  Air is forced through the front of a "Pitot" tube, sensing the pressure as compared to the side of the pitot tube, which senses the static pressure.  (Technically it's a pitot-static tube).  Basically it is like sticking your hand out of a moving car window and feeling the pressure of the wind on your hand.  The faster you go, the more pressure you feel.  That is also how much pressure the wing feels, and the airplane flies accordingly (faster = more responsive, slower = less responsive.  Now, if you stuck your hand out of a car going 100 kias at sea level, and compared the pressure on your hand to putting it out of the same car at 100 kias at, say 30,000 feet altitude, the pressure on your hand would be about the same.  However, keep in mind that the air is extremely thin up high like that (molecules are much more separated).  Therefore, you would TRUEly have to be going much faster through the air up high, to "feel" the same pressure on your hand (due to the less dense air up high, compared to the more dense, requiring you to be going at a much higher true airspeed to read the same speed in KIAS, or feel the same pressure on your hand.  This brings us to the next airspeed indicator in the cockpit...

2. KTAS, or knots TRUE airspeed indicator.  The KTAS is actually the true speed of the airplane through the air mass, at whatever altitude you are.  Today, for example, I was indicating both 265 knots KIAS and 495 KTAS at 37,000 feet on the way home from PVG.  So, if I had stuck my hand out of the window at altitude, the air would have pressed against my hand with a pressure the same as if I had stuck my hand out of the window at 265 KIAS on the ground, but in actuality, I had to be going much faster at altitude  (495 KTAS) to hit the same number of rarified air molecules with my hand.  Got it?  Think that's about it for the airspeeds indicated in the cockpit?  Think again.  Why did it take me three hours longer, at the same KTAS, same altitude, same route to fly from SFO to PVG, then to fly home from PVG to SFO?  Yep, you got it!  The whole air mass was moving (wind).  The wind was blowing west to east at about 120 knots, so my GROUND SPEED was different in each direction, which brings us to another speed indicator...

3. Knots Groundspeed.  Today, I was showing 265 kias, 495 ktas, and 615 knots groundspeed.  If I had been flying in the other direction, I would have been showing 265 kias, 495 ktas, and 375 knots groundspeed.  Note the difference in the speed across the ground is 240 knots (twice the wiindspeed, since you add the 120 knots of wind when it is behind you, or tailwind, and subtract the 120 ktas of wind when it's in your face, or headwind) to your speed when you.  If you have trouble picturing this, think of it this way.  You are swimming down a river.  You are swimming at 2 miles per hour.  The river is also flowing towards the ocean of course, at, say, 2 miles per hour.  Now if you are swimming downstream (with the current) someone standing on the shore would see you go by quite fast, at what appears to the observer to be about 4 miles per hour (your 2 = the river's 2).  Now, if that observer had big tits, and you decided to turn around and swim towards her at the same 2 mph, she would laugh her cute little butt off, since you would never get to her.  Your "ground speed" would now be ZED!  You’re going 2 mph through a current that is going at 2mph in the other direction, canceling each other out, so you would stay still with respect to the babe; even though you were working your butt off, you would never reach her.  Story of my life (until I met Kay).  Headwind, tailwind has the same effect of subtracting or adding to your True Airspeed to get your Groundspeed as does the current in the river.  Is that about it now for speed indications in the cockpit right?  No.  There is one other factor that comes into play as you go really fast.

4. Mach speed.  Actually it's simply your speed with respect to the speed of sound.  If you are going half the speed of sound, you are Mach .5, three-quarters the speed of sound, and you are Mach .75, the speed of sound, Mach 1.  Why does this matter to say, a commercial airplane driver?  Well, physics dictates that one significant quality of air changes at Mach 1.  It is called compressibility.  Below Mach 1, air is incompressible.  That means that as you fly through it, it simply moves out of the way for you, not stacking up on your nose, or compressing.  At Mach 1 air becomes compressible.   Therefore near or above Mach 1, your aircraft has to squeeze the air into a shock wave, rather than just letting it flow around you.  This compression, of course takes a whole lot of energy.  That means, a whole lot of drag.  That, in turn, means you must burn a whole lot more fuel.  You can actually double (or more) your fuel flow just to go another 5 knots "closer" to Mach 1 (which is very important when you need to conserve your fuel to fly a long distance, like us airline pilots).  Why does this happen when just "closer" to Mach 1, and not just after Mach 1?  Because when your airplane is flying at any one particular speed through the air, the air is actually moving over the surface of the airplane at slightly different speeds.  The air must go faster over curved surfaces, than over flat (streamlined) surfaces.  That means, the air that is flowing over say, the thick wing, then the air is flowing over the thin horizontal stabilizer.  So, at airplane speeds of about Mach .80 in the 747, there is some point on the airplane surface where the air is at Mach 1, or compressible, therefore adding to drag, and screwing up your fuel flow.  It's ok up to a point (worth the time saved) but as you speed up closer to Mach 1, the drag increases to a point where it is no longer worth it (in terms of fuel burned).  That speed in a 747 is Mach .86.  In the 777 it was .84.  In most Airbuses, it's around .81.  Oh, have I mentioned that Mach 1 speed is different at different altitudes because of the different temperatures?  Well, that's too far above the scope of this lecture.

So, in summary, today I was flying at 265 KIAS, 495 KTAS, 615 Knots Groundspeed, and Mach .84 (I was conserving fuel because I was early with the tailwind).  That means that in the cockpit, there were about a dozen indications of four speeds in the cockpit.  Each one meant something different, and would be used for different reasons.

Wheww.  That's about all my exhausted pea brain can handle.  I need a beer."


The outer ring is 13 miles long and 7 miles wide.
BNN = N 51 43.6 W 000 33.0

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