When gusty wind got the better of a taildragger on takeoff one recent day, local news reported that the ensuing collision with an element of runway infrastructure sheared off a main gear leg, but resulted in no injuries to those aboard.
Later that day, area pilots saw this notice to airmen: "09/011 - RWY 31 PAPI U/S. 18 SEP 21:54 2014 UNTIL 02 OCT 20:00 2014 ESTIMATED."
Good to know that the precision approach path indicator was now U/S (unserviceable)—especially if a pilot might have to execute an instrument approach procedure to that runway after prolonged flight in cloud. Breaking out beneath a ragged overcast in still-murky conditions, the instant confirmation of an on-glidepath indication reduces pilot workload, and adds certainty to the transition to visual flying.
What a pilot gleans from a PAPI’s white and red lights depends on other design features of the approach, reviewed and noted during preflight planning. On a runway served by an ILS approach, for example, the angle of a visual approach slope indicator (VASI) "normally coincides with the electronic GS angle." That angle is normally three degrees, but can steepen to 4.5 degrees for obstacle clearance. A PAPI is similar to a VASI, but "a pilot on the correct glide slope will see two white lights and two red lights," explains the Instrument Flying Handbook.
Glide angles are published for approaches with vertical guidance; for example the glide path provided by the RNAV (GPS) RWY 5 approach to Maryland’s Frederick Municipal Airport is 3.04 degrees, with a threshold crossing height (TCH) of 40 feet. (If a visual glideslope angle differs from the available electronic guidance, a note states that the two are "not coincident.")
About that TCH: Cite the Pilot/Controller Glossary during your next proficiency review and explain that TCH is "the theoretical height above the runway threshold at which the aircraft’s glideslope antenna would be if the aircraft maintains the trajectory established by the mean ILS glideslope or the altitude at which the calculated glidepath of an RNAV or GPS approaches."
A related term is the touchdown zone elevation (TDZE) figure; it is the highest elevation within the landing surface’s first 3,000 feet.
Wait—Frederick's VOR-A approach provides no TDZE. Why not?
TDZE is only used in the development of straight-in landing minimums. The VOR-A approach does not meet requirements for a straight-in approach.