Good News, Bad News: Uphill Headwinds
You and your Cherokee are at a 2500-foot runway having a significant slope, where the elevation of the high end of the runway is 1900 feet, and at the other end, it’s 1812. It’s a nice, mild day in the early spring, and the temperature is about 60 degrees. The wind speed is 15 knots, right down the runway. The trouble is, it’s really down the runway! Which would be best, taking off downhill, but with a hefty tailwind, or uphill, into the wind? There are 50 foot trees at either end (of course). How do you decide which way to take off?
- You don’t. Wait until evening, when the wind dies down, and take off downhill.
- Take off downhill, being sure to configure the airplane for a short-field takeoff.
- Take off uphill.
- There isn’t enough information to answer this question.
Answer: You may have asked yourself this question, somewhere along the way in your flight training (or afterwards): Is it better to take off uphill with a headwind, or downhill with a tailwind? (And even before being in this potential pickle, is it better to land downhill into a headwind, or land uphill, with a tailwind?) Just like the previous question regarding headwinds and tailwinds, there are other rules of thumb. One says that for down-sloping runways, you decrease takeoff distance by 5% for each degree. Another says that for up-sloping runways, every 1.0% grade has the same effect as an approximate 10% decrease in runway length. Yes, it’s confusing. And yes, there are many other variables involved. If you’re taking off downhill from a runway that’s only 1000 feet long, but the other end is 50 feet lower, then it’s like you had about 1150 feet. Of course, it gets complicated if the wind is also blowing downhill, too! Add in all the other “real world” factors, and this academic exercise can get pretty useless. It would also depend on the location of any terrain (particularly rising terrain) along your departure path. And we’re assuming the runway has the same slope along its entire length (no bumps or dips…like you’d probably find in the real world). Let me elaborate on this. (It’s worth the time.)
Any runway with a slope greater than or equal to just one third of one percent (actually, it’s 0.3 percent) is noted in the Airport/Facility Directory (with the direction of the slope, when it’s available), and on Instrument Approach Procedure (IAP) charts. First, on landing downwind: As already noted, a 10% increase in groundspeed gives a 21% increase in landing distance; even slight tailwinds noticeably increase the landing roll. If that runway is on a mesa or along a river, and you’re landing uphill with a tailwind, you could get an updraft on short final, and you could find yourself floating further down the runway before you can get it down. Also, when you land with a tailwind, you have to effectively fly steeper approaches to compensate for your increase in groundspeed, and this can cause illusions that can make it harder to judge your position relative to the runway. Then, as you’ve no doubt also figured out, taking off with a tailwind also results in a flatter climb-out, making it harder to out-climb rising terrain. As far as landing downhill into a headwind, first of all the wind is going to need to be strong to cancel out that downhill slope, right? That can mean more turbulence on the approach, especially with uneven terrain. You might wind up having to fly the approach faster to compensate for gusts, and the increase in your groundspeed will of course increase your roll-out. Also, landing downhill, unless you have done it many times before, your airplane will probably float forever, because the ground will seem to keep dropping out from under you! Once you are on the ground, I hope your brakes work really well; you’ll have an effectively heavier airplane, which might be rough work getting stopped in time.
There is actually a formula to determine the right thing to do. (And again, there’s still another old rule of thumb: This one says that for most small airplanes, it generally will take about a 1.0% uphill slope to counteract the adverse effects of each two to three knots of tailwind–or, put another way, you’d need two to three knots of headwind for each one percent of a downward sloping runway.) There are actually formulas that address this, quantitatively. There is a very complicated one by Herrington, et. al. (Flight Test Engineering Handbook, AF Technical Report 6273, Air Force Flight Test Center, Edwards AFB, 1966.) Here is a simpler one, although it is still an approximation, by John Lowry, a noted aviation physicist and author of many articles ( as well as a book) on airplane performance. The formula quantifies a wind velocity, which he calls the “break-even wind speed” which is the wind velocity that would give you the same distance to lift off in, whether you went uphill or downhill. Here it is:
The angle theta is the runway up-slope (in degrees); d lo is the POH “no wind” takeoff distance on a level runway (that is, for whatever the conditions are at the time, of course); and V lo is your lift-off speed in KTAS. This runway has an 88-foot drop over 2500 feet, which means… let’s see… the arctangent of 88/2500… that’s a tad over two degrees. (Something to tuck away in the back of your mind: a 35-foot elevation difference between opposite ends of a 2000-foot runway gives just about one degree of slope. A one-in-a-hundred slope, say a 21 foot difference for a 2100-foot runway, is about 0.57 degree.) Let’s say that your liftoff speed is 55 knots, and your POH says that you would need 1200 feet at whatever the particular density altitude was, at that moment. Plugging all that in says that the break-even wind speed is about nine knots. Since the actual wind is well over that, taking off uphill to take advantage of the wind is actually the right thing to do. The right answer is choice C.
If you hear a repeating “dit-dah-dah-dit” (the letter “P” in Morse code) on 121.5 MHz, what kind of device is transmitting the signal?
- a 406 MHz Personal Locator Beacon (PLB)
- Actually that’s two letters, being the Morse code for the letters “A” (dit-dah) and “N” (dah-dit) from an outdated (but still operational) radio range.
- Over-flying an outer marker beacon (75 MHz) will cause any VHF radio to pick up a harmonic of its 400 Hz audio tone on 121.5 MHz, which pulses at 300 milliseconds and sounds like Morse code for the letter “P” (even though it isn’t).
- The “P” code from your GPS can be heard on your comm radio on 121.5, if the squelch is turned up fully.
Answer: It’s a low-power homing signal, transmitted by the PLB to help searchers with 121.5 MHz direction finding equipment in locating the PLB–and its user. (Why the letter P? I don’t know, but it was suggested by the National Telecommunications and Information Administration. It might just be because that’s the first letter in “PLB”.) Anyway, PLBs will be authorized for use throughout the US under Part 95 of the FCC Rules starting on July 1, 2003. (Canadian-approved PLBs are already in use in Alaska, under an FCC developmental license granted to the state.) As far as choice B, with the old four course radio range, first in use over seventy years ago, the user would hear either an “A” or an “N” or if he was “on the beam”, a steady hum. Also, they used much lower AM radio frequencies, so you couldn’t possibly hear anything on 121.5. Choice C is nonsense, as you wouldn’t get radio frequency interference on a frequency that isn’t an integer multiple of 75 (which 121.5 isn’t) and even if you somehow had crossed wires with your marker beacon receiver, you would hear dashes (a Morse code “T”), not short and long pulses. (The inner marker though, modulated at 1300 Hz, does use alternating dots and dashes, which one could say is either an “A” or an “N”.) Choice D is equally goofy, as the P-code is binary; it’s not an analog signal. It’s also over one gigahertz, and so is not modulated on a VHF carrier! It’s A, folks. See: FCC Word Document
The first aviation law dates back to:
Answer: The FAA (nee Federal Aviation Agency) was “created” in 1958, but the first US aviation laws were enacted by Congress in 1926 (the Air Commerce Act). The International Convention for Air Navigation in Paris in 1919 established basic principles of air sovereignty, required national registration of aircraft, placed restrictions on movement of military aircraft, established a body of basic rules of airworthiness of aircraft and pilot competency, and provided for police measures. Even earlier, in 1907 (October 18, to be exact), the Second Peace Conference at the Hague established an edict (later conspicuously ignored) which prohibited the dropping of bombs from airplanes. Even more amusingly, on July 17, 1908, the town of Kissimmee, Florida passed a statutory regulation which required aircraft registration as well as aircraft “behavior” (speed and altitude) within municipal limits. On June 8 of 1911, Connecticut created regulations governing aviation (becoming the first state to pass such laws). Many other early laws, such as the prohibition of flying on Sundays (England) were enacted, as well. (One such law supposedly applied to Joseph and Jacques Montgolfier soon after their first balloon flights, but it has not been substantiated.) It’s C.