Feather-tech · 4:05am Jul 14th, 2014
It's been suggested to me a few times now that, since The Last Link is based heavily on real-world technologies and their practical limitations, it would behoove me to explain a bit more about some of the stuff I reference. After all, it's been years and years since anyone had to deal with a non-crystal-locked radio, let alone Morse code navigation. So here's the first of at least a few posts that'll dig a little deeper into what Featherprop, Pasture, Ether and Espresso have to deal with. If people like them, I might get into some other ideas that have been floating around, or some character history that doesn't quite fit in the story.
Q: What is a "range" and how does it work?
A range is, at it's core, just a set of low-frequency radio transmitters. There are two antenna sets, arranged in a cross configuration, that each transmit the same Morse code character to diagonal quadrants. One antenna transmits the letter A, dot-dash, while the other antenna transmits the letter N, dash-dot. Imagine a four-leaf clover with leaves that are diagonal from each other given the same label, A or N. The leaves would represent the lobes of radio signals broadcast by the antennas.
The most important feature is that these lobes overlap slightly; instead of a sharp transition, the A and N tones will gradually blend together. When an aircraft is centered on the proper flight path, the A and N tones combined to form a continuous tone. An experienced operator can even estimate distance from the range based on how wide the zone of blending is- the closer to the station, the narrower it gets and the harder it is to stay centered. Unlike more modern systems, staying on course was an art form, with lots of trial-and-error to estimate the correction needed to compensate for wind and changing distances.
The system is simple and reliable, but also limited in some important ways. The most significant is that, when you are nowhere near the blending zone, you cannot tell where you are-- all that can be determined is that the aircraft is in one of two diagonally-opposed quadrants. Until the aircraft crosses a boundary between lobes, it cannot even tell which direction it is from the station. The system is also subject, like all radio frequencies, to certain forms of interference. While low-frequency signals carry a great distance, they are also more susceptible to interference from atmospheric conditions, such as lightning or aurora activity. Another major limitation is that, being aurally tracked, it was necessary to change frequencies to track the next navigational range, meaning that there would be a period where the aircraft was potentially drifting off course, and of course the chance for the pilot to get confused and misinterpret the new set of signals.
Beyond cross-country navigation, the ranges can also be used for blind let-down/instrument approaches. Notice that many are set up several miles from airports, rather than directly on them. This accomplishes several purposes. The distance allows for the pilot to determine his exact point in space as he passes over the range, so that he can descend further without worrying about flying into terrain.
A common feature of all range-based approaches is the use of time to substitute for the inability to accurately measure distance. When starting the approach, the aircraft flies outbound on the beam for a certain time, then performs a course reversal, always to the "protected" (safer) side of the beam, re-acquires, and tracks it in to overfly the range.
Often, an initial descent occurs once the aircraft starts the approach, to reduce the need for a rapid descent later. Once crossing the range a second time, there's usually a descent to a final altitude (dependent on aircraft speed, the runway in use and possibly other factors), and the timer is once again started. Measuring time from the navaid is essential, because the final altitude is only safe within a certain radius of the airport. If the time limit is reached and the pilot cannot see any part of the runway, the missed let-down procedure must be followed, climbing and often homing back in on the range to either try again or proceed to a different destination.
Most, if not all, were sited so that one range would pass over or very near the airport, in some cases so it is perfectly aligned with a particular runway, allowing a lower approach because there's no low-level maneuvering needed. These charts originated as simple notes and drawings which were then codified. In compact form, they provide frequencies, a map view, a plan view, and some very important information at the bottom: The minimum altitudes and visibilities needed to conduct the approach safely.
Next time on Feather-tech: Icing and what's so special about that orange and green goop they spray on the airplane. "Type I, it's what candy would taste like, if de-icing fluid was a candy."
I can remember cross-country across Kansas to Denver using only Non-Directional Beacons, maps, and a compass in 1980. Most of the destination/departure airports had radio stations, so unless the wind was really nasty, you could stay 20-30 miles within where you wanted to be no prob. I still have my old flight computer in a box downstairs, even though I could never figure out how to use it now, and any of the newbies would just look at it and say, "Dude? Really?"
Ah, memories. Flying back from KC in the dark with icing at 3,000 ft and ground at 1,000 ft. Oh, and some 1,000+ ft radio towers in the flight path too. Flying back from Wichita with a dead alternator, no radio or lights, and in the dark too. Flying to ABQ in the snow and having to turn around and land before the ceiling and the ground met out there in front of us somewhere.
You know, there's a reason I don't fly any more...
A bunch of your pictures are just the url.
Still, I, for one, am all in favor of you continuing these blogs.
Feel free to add personal experiences where applicable, too. We all love living vicariously.
I remember reading a book about early experimentation in range finding during World War II. I believe it was by Arthur C. Clarke. I don't remember all the specifics, but for one of their final proving tests, they needed to obscure the runway to demonstrate that planes couldn't land visually.
They did this by setting smokey fires all around the runway, apparently completely forgetting what kind of insane updrafts that would cause. The demonstration was less than a stunning success, but none of the pilots crashed.
Hmmmm.... I think I might have heard a bit about icing before on Air Crash Investigation....
If I'm not mistaken, icing is the phenomena which happens when water vapour condenses on aircraft surfaces and after that, freezes due to cold temperatures (which means that it won't happen on ground in a tropical area or during summer). Icing may also happen in mid-air. According to my limited aviation knowledge, ice on the airframe may cause changes in airflow along the entire airframe. Aircraft are designed in a way that their best performance is achieved when the airframe is as is. Since aircraft rely on aerodynamics to fly, any changes in how air interacts with the aircraft will cause serious performance issues with the aircraft.
The goop they spray on aircraft prior to departure is de-icing fluid. Now I don't know about the different types, but the fluid causes the ice to break off the airframe during take-off. (My guess is that the fluid changes the chemistry of the ice, making it more fragile.)
Pardon my limited aviation knowledge, I just want to see how much I know about it. If there's something wrong, can you correct me?
Oh, and I had absolutely no idea about ranges until I read this
2282667 Er, yeah, I just caught that, and fixed 'em. Sorry 'bout that!
Geez, where could I start with some? I haven't done any AN range approaches, but for a while I regularly did some NDB approaches. We didn't follow the procedure, exactly- due to a rather large mountain behind the airport, our procedure was hit the beacon, chop and drop, and if you didn't see anything at the bottom, you threw the coals to it and climbed at Vx. It was always freaky if you saw what was below you during the course reversal- there was one tall hill that looked waaaay too close.
2282782
Heck, pretty much anything. I've found that my serious readers seem interested in nearly any subject I care to broach.
On the topic of aviation, we haven't all been blessed to be up in a small plane (my brother let me fly his once. I could maintain course or altitude, but not both). He's clipped trees on takeoff, and we nearly ran out of runway in Flushing. I reassured myself that he knew what he was doing, and we weren't going to hit that hanger . . . afterwards, he said, 'you know, I didn't think we were going to stop before we hit that hangar.' I said, 'I'm glad I wasn't panicking for nothing.'
EDIT: you could also mention the giant cement arrows that are on some of the air routes out west in the US. That seems like a pony-style solution to aerial navigation, and it's an interesting look into American aviation history.
2282665 Holy crap, dude. That's some hard-core stuff there. I've done a ferry flight nocom, but that was years ago for a flight school. One time I DID take off into icing without getting the generators online- I only caught on when the radios started cutting out and looked at the battery voltage. I still had enough volts to excite the generators, so I could keep going, but it had me on edge.
Cold weather can catch you in surprising ways. On a -20F day, just as I hit V1 my airspeed indicator froze up- the cold air coming in the pitots froze some water in the line. Just as I entered the clouds at 400ft, the second ASI froze up, too. Since I was already in the clouds, I figured it didn't make much difference whether I circled around or proceeded. Like most super-cold days, there was a massive inversion, so I hoped the warmer air up above would let the line clear. It worked- after half an hour, the right ASI came back, and just before descent the left one did too.
Also, the E6B is the greatest tool ever. I use my Dad's old AF issue one every once in a while, just to stay in practice.
2282714 Ranges are almost the old-schooliest of old-school nav- the only thing older is the airway beacons. There literally used to be a network of beacon lights stretching along the air routes that would be lit up at night to guide aircraft. There's also some great information on the ATC History page, and a list of beacon sites here
And you've got some of the basics of icing; I'm writing some stuff up and grabbing some images for another blog post, since it's really more than I can toss up here. Then again, modern de-icing is a bit beyond what Featherprop and the other FFS pilots would use; if they're lucky, they'll have a hangar. If not... wait for the freezing precip to stop, then sacrifice some of the low-grade hooch. ... I'd better stop before I start with the colorful anecdotes that'll fit well with the whole post!