Slightly Higher In Bolivia
Airliners usually maintain cabin pressure altitudes somewhere between about 5000 and 8000 feet during cruise flight. But just how does the pressurization system cope with situations where they’re landing at an airport with an elevation higher than that? (There are several. La Paz, Bolivia, for example, at 13,313 feet above mean sea level.) What does an airliner do, when it’s flying along at FL 330, with a normal cabin altitude of say 7000 feet, but it’s landing where the terrain itself is significantly higher?
- During the climb to cruise altitude, cabin pressurization is gradually adjusted, up to and then beyond the usual cabin pressure altitude (but more slowly), until it has reached the expected pressure at the higher elevation destination airport.
- Cabin pressure altitude remains at no higher than 8000 feet, despite the higher landing elevation, because of operational and engineering standardization in most airliner pressurization systems. In addition, Part 135 regulations would require that at least one of the pilots on “flight deck duty” wear oxygen masks continuously, (and for Part 121, that’s all crewmembers) whenever the cabin pressure altitude exceeded 12,000 feet. Shortly before the doors are opened at the gate, outflow valves allow a fairly rapid (but not explosive) decompression to ambient pressure.
- Whenever an airliner lands at any such high elevation airport, the cabin is actually “depressurized” to that pressure altitude (from the usual cabin pressure altitude), but rather than doing this at the arrival gate at the last minute, it is usually done during the approach to landing. (The oxygen-related regulation mentioned in the previous selection may still apply.) In the case of extreme examples such as La Paz, the cabin pressure altitude can also be raised to 13,300 feet while enroute, at whatever flight level is chosen or assigned. Once the cabin is depressurized to that altitude, the crew maintains that cabin altitude. This is so that upon landing, cabin altitude and actual elevation are the same. The outflow valves automatically open upon touchdown to eliminate any possible residual pressure that might prevent the doors from opening (although in some installations, there are automatic pressure controllers which gradually decrease cabin pressure altitude during the approach, rather than the more rapid drop via outflow valves).
- Most airliners serving such high (and usually remote) airports do not actually fly at higher flight levels that often, and they simply never pressurize the cabin at all. Also, in most of the countries having such high elevation runways, our FAA rules don’t apply, and the crew on such flights are themselves acclimatized to the thin air.
Answer: Enough blabbing already–the correct answer is choice C.
World’s Fastest Helicopter
The world’s fastest helicopter could travel at
- 150 knots
- 216 knots
- 263 knots
- 630 knots
Answer: C. Airplanes stall when they get too slow. With helicopters, it’s just the opposite; they can actually stall because they are going too fast! Why? The answer can be inferred from the name of the phenomenon: retreating blade stall. All helicopter manufacturers must establish maximum airspeed limits that are below this threshold. For example, the rotor tip speed in an Robinson R22 at 104% rpm, or the “top of the green” on the tachometer, is somewhere around 370 knots. The helicopter’s “never exceed” speed is just a bit over 100 knots (102 kt, at sea level). Even with the greater angle of attack that is allowed by the flapping of its underslung and teetering pair of blades (or about the flapping hinge, on articulated rotor systems), there still comes a point where the relative motion of the “retreating” blade is below that needed to remain level (even with the blade on the retreating side having a greater angle of attack), and this dissymmetry of lift would cause an unfortunate helicopter to both pitch and roll over. This is not a good thing. (The “roll” part comes from that dissymmetry; the “pitch” is due to gyroscopic precession.) Helicopter manufacturers publish charts or graphs showing Vne decrease with altitude. Retreating blade stall is aggravated by high weight, low rotor rpm, turbulence, or abrupt maneuvering, as well as (of course) excessively high forward airspeed. Unlike airplane pilots, helicopter pilots often think in terms of their “percent rpm”. At “full” rpm (which with the governor on, is held at approximately 104%), as well as in forward flight right at Vne, the percent drop in tip speed on the retreating blade’s side would be about 70%. (Note that this is at the faster-moving tip; it’s much worse towards the middle.) This just happens to correspond to the percent rpm below which a helicopter experiences what is known as catastrophic rotor stall. Beyond that point, there is no recovery, even by fully lowering the collective. The rotor blades rapidly slow to a near stop, and the helicopter falls straight down, right out of the sky.
Although the Westland Lynx (choice B) can hit over 200 knots, the Lockheed XH-51A could top 250 (263, or just over 300 mph). In addition to the engine powering the rotors, it was also powered by a single J60-P-2 jet engine mounted on a stubby left wing. A number of helicopters can easily top 150 knots (choice A), but nothing with rotating wings can get anywhere near 630 knots. (Even the V-22 Osprey tiltrotor has a top speed that is, at most, about half that.)
Getting the Red Carpet Pulled Out From Under You
What IFR aircraft under ATC guidance in IMC (instrument meteorological conditions) may suddenly find that their services have been terminated, and that they have to fly a full instrument approach, rather than simply being vectored to the final approach course?
- any aircraft
- any aircraft, when radar coverage is no longer available
- any aircraft except military aircraft on Department of Defense missions
- any aircraft, unless the President of the United States is aboard
Answer: For those who aren’t familiar with instrument approach procedures, the deal is that, generally speaking, a pilot in instrument conditions flies to this fix or point in space called the Initial Approach Fix, and then goes outbound away from the airport for a couple of minutes, possibly descending to some lower intermediate altitude. He or she then makes a controlled course reversal, and flies in toward the airport, intercepting whatever electronic guidance is available there, descending to a minimum altitude or decision height. In the real world, the vast majority of the time in controlled airspace at least, pilots get help from ATC in the form of “radar vectors” to intercept that final approach course at an angle which is small enough to preclude their having to do all that course reversal and turn-around jazz, and it saves time (and money).
As for the answer, nope, it’s not D; even the President isn’t exempt. This has actually happened, too (on October 21, 1988, in one particular case). Frederick, Maryland, home of AOPA, is also the designated reliever airport whenever the ceiling and visibility are so bad than even the Marine One helicopter can’t get the President into Camp David. In such cases, then Air Force One for example will do the ILS 23 into Frederick, and the President would continue the last dozen or so miles by car. Even though nearby Dulles airport has radar that can see a bit lower than the other nearby major airport, BWI, it is Baltimore Approach which is the controlling facility. (It may be due to the fact that Baltimore was designated as the controlling facility for FDK even before Dulles existed. This is a local political battle that has raged for years, but which is probably going to be history when the consolidated Potomac TRACON is fully operational.) As the story goes, Dulles radar controllers could see down just about into the pattern at KFDK, while Baltimore’s radar wasn’t reliable below about 4000 feet! (This has improved somewhat since then, thanks to ASR-9 radar.) But when you drop off the scope, forget radar vectors to the final approach course, even if your plane is sporting the Great Seal (or another non-designated facility can see you). You’re on your own. The answer (finally) is B.
