Credit to the photographs

I would like to thank Brian Losisto (Air Canada's photographer) for always allowing me to post his pictures. (The above thrust lever pic is his). Then there is Kelly Paterson from Calgary and plane spotter "Erik" from Germany. Of course, I have lots myself. On that note, if you feel a photo(s) may be in appropriate or the content I post a bit dubious by all means send me an email. I will ratify it! That's all I ask!
P.S I'd like to add Nadia from "la belle province" for her contributions!
...and YYC Disptacher...

...I hope you enjoy the blog...

Tuesday, November 3, 2009

PAPI (Precision Approach Path Indicator)

From follower: Lakotahope:
PAPI (Precision Approach Path Indicator). Ok, maybe I am dating myself with this question. VASI (Visual Approach Slope Indicator), I remember, but the PAPI is unfamiliar. I am gonna have to start all over when I get back into the air.

Lakotahope: Seems like more and more airports are going the PAPI route instead of VASIS.
They are located near the touchdown point and work on the same basis as a VASIS. The two inner lights (closest to the runway) should be red and the two outer lights should be white.
Even though my shot on final into Heathrow is a little blurry, you at least get to see two red and two white lights on the left. Similar to VASIS it provides a 3 degree slope. When you see all four lights white - you are too high. All red, you're too low or as the saying goes, "all red, you're dead!"

Many times when we follow the glideslope down the PAPI (VASIS) can be off a bit. The question is, "which one do you believe?" The approach onto the 15s (15 left and 15right) in Toronto come to mind as far as poor agreement.


david said...

I'm not an expert on optics or radio waves, but I'd guess that the optics would be less subject to distortion, assuming both are set for the same glidepath.

On the other hand, Nav Canada has already taken any GS inaccuracies into account in setting up the approach, so maybe it's a bit like altimeter settings -- you might be at only 9,700 ft MSL when your altimeter says 10,000 ft, but as long as everyone else has the same error, you're fine (barring any mountains).

From the Flight Deck said...

Good point David. Maybe it's an Airbus thing? It happens during the last two to four hundred feet where things don't always agree. It's an observation I've made over the years. I guess it's not a perfect system. Doug

david said...

Doug: after a lot of puzzling, I think I've figured out why the glidescope seems to fall out from under my plane for the last few hundred feet of an ILS approach, no matter how well stabilized the rest of the approach is.

Close to the ground, the wind speed typically drops a lot. If I'm flying into a headwind (as I should be on an ILS), that means that my glidepath will suddenly get much shallower at the same rate of descent. Bingo -- I'm above the glidepath.

From the Flight Deck said...

David. Looks like you are putting lots of energy into this.

Wind may play into, but vertical speed would be adjusted accordingly. If I was descending with a strong tailwind, the vertical speed would be greater. So much so, most airplanes would start yelling at you by saying, "descent rate!" For a strong headwind the descent is much shallower to maintain a typical three degree glideslope. The PAPI is theoretically projecting a three degree profile as well, of which the pilot adjusts accordingly due to winds. But I think the variability of winds closer to ground (your answer) and the sensitivity of the systems closer to touchdown are the culprits.


brian.crissie said...

is that a Concorde on taxi?!

From the Flight Deck said...

Brian. Good eye! It is indeed a retired British Airways Concorde.
They have it parked on an abondoned runway. I have a picture of it in my book.

Again, good eye becuase the photo is blurry.

Just laned in YYC (Calgary) inbound from Toronto.

Tomorrow it's off to Vegas with a layover in YVR (Vancouver).