It was a challenging flight, and that hectic last vector to final didn’t help stabilize your approach. Now, as you pop out beneath a ragged overcast, the runway remains a murky outline up ahead, but the VASI alongside helps you complete a transition to visual references. Nothing like that good old two-bar visual approach slope indicator—the one your primary instructor didn’t want you to depend on when you were learning to land an aircraft—to comfort you now with its on-glidepath indication.
Visual glideslope indicators are a huge help during the endgame of an IFR flight, once you make the commitment to complete the arrival visually. No more intense concentration on timing to a missed approach point; no more stress about a possible miss in murk at minimums. Just keep the colors right, or the elements aligned, depending on what kind of visual system exists at the destination.
A precision approach path indicator (PAPI) is the system in use for Runway 17R at Oklahoma City’s Will Rogers World Airport. It’s also the preferred path down to Runway 17R at Wiley Post Airport, a few miles north. The two PAPIs have something else in common: They are both "P4L" systems, that is, "4-identical light units placed on left side of runway."
The coincidence of such similar systems installed on runways with the same magnetic bearing only a few miles apart wasn’t foremost in the minds of an air carrier crew when they picked up the light beam during an arrival from the north.
"Upon reaching 2,800 feet the pilot not flying commented that the ILS DME for Runway 17R still showed 12 NM, but that the runway seemed very close. We then both commented that the ILS glide slope showed low while the VASI showed high," said a crew member in a report to the Aviation Safety Reporting System (presumably using the term "VASI" generically in the report).
The crew credited their growing awareness of the contrary indications to their company’s policy of "backing up all approaches with available navaids"—good practice for any pilot.
The conflict was confirmed by the tower, all perhaps averting a landing on Wiley’s 5,002-foot-long runway instead of Will’s 9,800 feet of grooved concrete on the south side of town. The flight climbed back to 3,000 feet, "picked up OKC 17R and landed without incident," the crew reported.
Speaking of subtle distinctions, the Oct. 7 IFR Fix: Probably just the gauge challenged readers to locate a minor typographical error on this instrument approach plate, offering a hint that "it’s a small thing that may be hard to spot." The typo was the omission of a space between "may" and "be" that still appears on the new version of the IAP valid from Oct. 17 to Nov. 14 in this note: "use of BKW R-175 and BLF R-045 maybe necessary to locate DROWE intersection."
Reproducing the note verbatim is causing the word processing program used to create this article to flag the typo—confirming how valuable using "all available systems" may be.