All,
The 67% runway left discussions refer to PREFLIGHT, dispatch planning only. Upon arrival, you can "plan" to use ALL the runway if needed under the conditions, even if maximum stopping performance is required to have one foot left. (Not smart, perhaps, but legal). One "should" have some solid margin left upon arrival, though, BECAUSE OF that preflight planning requirement; which is the whole point of the preflight planning FAR with tons of "slop".
Approach end displaced threshold is NOT considered in landing distance math. TCH's are, therefore, AT the "threshold" where green lights and paint are. Displaced thresholds on the approach end are included for takeoff calculations only. Departure end displaced thresholds do "count" for landing and takeoff abort distances.
SWA OPC calculations assume ALL landings touch down 1500 feet from the "threshold" (not displaced threshold where takeoff roll starts). Following the HUD AIII mode WILL put you down 1500-2000 feet from "that" threshold unless "flare guidance" is ignored once visual. Hence, a 1000 foot additional margin is added to landing distance if "AIII" is selected for a real AIII low vis approaches with our 50 ft decision height. AIII published approaches only exist on long runways, so following flare cues in 700 ft RVR's is not a problem. It IS a problem at MDW. 31C ILS-Z is NOT an "AIII HUD" approach, but an "IMC HUD" approach (GS info goes away with no precision flare guidance at 35 ft). ILS-Z HUD only means IMC HUD (NOT "HUD AIII") and the new Jepps changes on the approach plate specifically disallow using the AIII approach. Heck, it is a CAT I approach requiring 3000 RVR! The special HUD use simply lowered the normal ILS DH from 250 ft AGL to 200 ft AGL in order to pick up the LDIN and threshold lites earlier.
It is EASY, and IMO, proper to "retarget" to the 500 ft point under such conditions using the HUD if one "had" to land at MDW for whatever reason (emergency, fire, etc). That gives one 1000 ft more stopping margin than OPC predicts!! Once visual at minimums, ease the HUD flight path marker (i.e. ground impact point!) onto the "real" threshold lites and drive in holding it there (on speed, of course) until the flare point at about 30 feet. If you do the trigonometry you get a 4.06 degree glideslope from 200 ft "on" the normal 3 degree glideslope (I.E. at normal DH) to the threshold (instead of to abeam the GS antenna at about 1000 ft from the threshold). There are no approach lites to "hit" anyway with the displaced thresholds. A 3 second or so flare "process" then makes you "miss" the threshold "impact point" and you safely touchdown around 500 ft down from it (with about a 20 ft TCH... no problem). IMO, this is perfectly safe, prudent, and legal by FAR's. It is “stabilized” also at just under 1000 fpm sink rate for average weights.
It is not SWA "SOP" to NEVER, EVER "duck under" or “retarget“ in any STRICTLY written verbiage (although it is in many pilot‘s minds). IMO, short, slick, runways make shortened “aim points” the SMART thing to do and there is wiggle room in "SOP" verbiage to do so (not to mention FAR's blatantly allowing it... see below). There is NOT an industry “land short problem”… there IS an industry “overrun” problem. If a 737 captain can't safely touchdown at the 500 foot point easily at any time you might need to -- IMO, that is problematic.
The regs allow a thinking pilot to do what is necessary after DH/MM glideslope-wise to ENSURE a safe landing. SWA removed (for unknown reasons... good, IMO) the short-lived 2004 revision language which stated we “fly the glideslope to the flare“ well before this accident. Here is the Reg:
FAR Part 91.129.e
“(2) A large or turbine-powered airplane approaching to land on a runway served by an instrument landing system (ILS), if the airplane is ILS equipped, shall fly that airplane at an altitude at or above the glide slope between the outer marker (or point of interception of glide slope, if compliance with the applicable distance from cloud criteria requires interception closer in) and the middle marker; and
(3) An airplane approaching to land on a runway served by a visual approach slope indicator shall maintain an altitude at or above the glide slope until a lower altitude is necessary for a safe landing.”
Respectfully,
GQ
Last edited by GregMagicQuist; 1st February 2006 at 07:37.