It came from Black Background: Matt Shobe's Blog

This is something I've been meaning to post since I returned from Hong Kong, but I simply haven't had the time to do so.

I flew United to and from Hong Kong with their nonstop service from Chicago. It's about 14 hours in the air, and it's the sort of flight plan that pushes up against the comfortable limits of modern commercial air travel -- even if you're in upgraded business class. I was sitting in the upper deck section (highly recommended for this trip if you can score the upgrade with miles) and before our noon takeoff I managed to get a few minutes with the pilots and flight engineer in the cockpit. (Paradoxically, the everyday security paranoia that permeates domestic commercial service is a bit restrained overseas; at the gate they kept the cockpit door open and seemed happy to chat for a couple of minutes right up until they got word that everyone was on board and the flight was ready for an early push-back.) After showing me the obligatory cockpit copy of "Flying for Dummies" with all the expected yuk-yuks, I asked them a few questions about their flight planning approach and why there was no air traffic control on Channel 9 when the flight neared the arctic circle.

I got a few interesting tidbits out of them:

1. The maximum takeoff weight of the fully-loaded 747-400 we flew is 875,000 pounds. Our aircraft took off at just over 874,000. (The pilot had me run the fuel weight calculation on my palm for him "just to be sure." I have a feeling they already knew the number by heart, but it was fun to play along.)
2. They don't know, at takeoff, exactly how much runway they're going to need to get off the ground. They have some idea of the minimum, but everything depends on air density, how much fuel they've burned at idle, and some other factors they didn't go into. I found this extremely odd, since you have all sorts of performance charts to tell you how even a lowly Cessna will perform at a wide range of temperatures, altitudes, and weights. Something about jets makes all the easy math hard. I think Hong Kong's runways are 13,000' or better, so it's really not an issue, but I still would've thought they'd have more certainty.
3. Channel 9 goes mute near the arctic because the pilots actually do position reporting using UHF radio (vs. VHF, which is in use at all the 'normal' latitudes). UHF is required to reach the nearest reporting stations somewhere in the icebox Yukon. Apparently, their UHF band is full of static and they'd just as soon spare the six of us listening to Channel 9 all that worthless noise. Thanks, guys.

When takeoff actually happened, I think we got off the ground at about the 9,000 foot mark. Knowing at that moment exactly how much mass that aircraft was hoisting into the low seaside overcast gave me some added appreciation for just how well designed (and piloted) commercial aircraft are today.

Once enroute, I decided to record some of the air traffic control communications near mainland China. I figured they would be different enough from the usual general aviation chatter I hear over Chicagoland that they'd be worth sharing. How did I record it? That was a fun hack to cook up:

1. I own a pair of Bose Series II Noise Canceling headphones. A modified-but-mostly-standard 1/4" audio plug connects the headphones to the standard stereo outlet found on most modern aircraft seats. The key is that the headphone end is removable, so the headphone cable can also act as a general purpose cable with two plug ends. I tuned Channel 9, plugged into the jack, and then plugged the other end into my Powerbook's Line In jack instead of the headphones.
2. I fired up Audacity, an excellent open source audio recording/remixing client. I probably could've used GarageBand, but Audacity was simply my choice at the moment.
3. With a new sound file open, I hit record, selecting "22k Hz mono" as my recording format -- ATC transmissions are over VHF frequencies similar to those you use to listen to AM radio, so recording in stereo is a waste of bandwidth.
4. I put Audacity into the background and let it record a few minutes of audio. I returned to the file later to double check the input level and found that it was surprisingly good -- I didn't have to tweak anything to get a smooth, normalized audio stream. Got my recording in one take!

I later edited the following file down from ten minutes to just under two minutes, saving the three most interesting exchanges (which are admittedly pretty hum-drum, but you pays your money and takes your seat for this show). Here's the audio:

(MP3, 1.6MB)