From: firstname.lastname@example.org (Ed Hahn) Organization: The MITRE Corporation, McLean, Va. Date: 03 Nov 95 02:51:38 References: 1 2 3 4 Followups: 1
View raw article or MIME structure
In article <airliners.1995.1701@ohare.Chicago.COM> email@example.com (Timothy Kono) writes: Robert Ashcroft (firstname.lastname@example.org) wrote: : In article <email@example.com>, : Michael Coch <firstname.lastname@example.org> wrote: : >There are pros and cons to everything. According to my pilot friends, you : >don't want to be in a light jet during landing if there's wind shear -- : >takes a little too long for the engines to spool up to counter the shear. : Is this true? I'm just wondering but, are lighter jets more likely to be unstable during such things as wind sheer? Also, would faster and more powerful engines counter the sheer much faster and better? I just want to know. _____ In a windshear encounter, there are a couple of things going on. (I'm actually going to talk about microbursts, since they are the most "dangerous" form of windshear.) First, there (usually) is a pocket or stream of cold air descending vertically from a thunderstorm cloud. When this gets near the ground, it turns out in the radial direction and accelerates to high velocities. An ASCIIgram of the cross section (use fixed width font for best results): ||| <-Descending air ||| /|\ \--/|\--/ <- flattening and "rollup" near the ground ( < 2000 ft AGL) ================= <- the ground In the typical "hazard" case, the aircraft is flying an approach only a couple of thousand feet above ground. What the aircraft experiences is first a headwind, then a sharp change (shear) into a downwind followed immediately by a tailwind. Since the aircraft cannot change ground speed fast enough to maintain a constant airspeed, it "appears" that the airspeed drops quickly (in a few seconds) by large amounts (30+ knots). Obviously, this is a bad situation for any aircraft, including propellor and jet aircraft. However, with jet transports, there is a second, more dangerous effect than just airspeed loss. An airframe in flight, like any dynamic system, has "modes" of motion. One mode which every pilot becomes familiar with when first learning to fly is the phugoid mode. It is a pendulum-like motion caused by an out-of-phase oscillation of altitude and airspeed. It just so happens that the typical approach speed of a jet transport and the size of the microburst end up exciting this mode of oscillation, which makes recovery extremely difficult. Furthermore, since jet engines have a spool-up time latency (even just a couple of seconds), this can be the difference between hitting the ground and not hitting the ground. It turns out the recommended "escape manuever" for jets in a microburst is to command maximum thrust and pitch to "stick shaker", or maximum angle-of-attack. This is because the aircraft needs to combat altitude loss as quickly as possible. For a slow piston-engine aircraft, the "escape manuever" is not as clear: it turns out that for some aircraft, it's better to INCREASE airspeed by pitching down in order to escape the downdraft portion of the microburst as quickly as possible. (Slower aircraft by their nature are less susceptible to the airspeed loss because the slower speed translates to a weaker shear, but instead spend more time in the downdraft part.) However, research continues in this area. Bottom lines: 1) smaller, slower aircraft are not as greatly affected by the dynamics of a microburst encounter compared to jets. 2) If you're on a jet in a microburst encounter, you want to have as much excess thrust as possible to combat altitude loss. Sorry for the long reply, hope this helps, ed -------- Ed Hahn | email@example.com | (703) 883-5988 -------- The above comment reflects the opinions of the author, and does not constitute endorsement or implied warranty by the MITRE Corporation. Really, I wouldn't kid you about a thing like this.