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Glide path lights

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General Aviation

Is glide path the same for all aircraft when it comes to vasi and papi lights?  For instance, if I'm flying a Cessna 182 and see two white lights and two red lights, I am on the proper glide path.  Would a Boeing 747 see two whites and two reds at the same angle/altitude as the Cessna?

How was proper glide path established in the first place?

As a side, can "glide path" and "glide slope" be used interchangibly?

2 Answers

  1. John D. Collins on Feb 15, 2011

    Read the AIM, section 2-1-2 on Visual Glide Slope Indications.  It has a good writeup on the various VASI and PAPI systems.  If you see two red and two white lights at a particular altitude and distance from the runway, then regardless of the aircraft type, your eyes will form the same angle with the lights.  That having been said, a large aircraft such as a 747 has a cockpit that is much higher when you measure it with respect to the wheels than your C182.  So often there will be a second higher glidepath to provide “high cockpit” aircraft greater wheel clearance when crossing the threshold.
    My understanding, and I have no written backup for it, is that when the original glidepath of three degrees was adopted as the normal, the air force would not agree to a higher glidepath because their jets would have to flown too fast with too little power at higher glidepath angles. The jet engines took a substantial amount of time to power up in the event of a go around, so operating at low power could jeopardize the go around decision. At the time, the airlines were still flying piston engine aircraft and didn’t have the same issue.  The three degree approach angle allows the jet to maintain substantial power and with the deployment of drag, are able to maintain a stabilized approach with a reasonable approach speed.  The C182 has no equivalent issue and can easily fly a stabilized approach at 6 degrees while still carrying some power to be able to adjust the glidepath.  For reference, most of our GA aircraft have a power off glide ratio between 7 to 1 and 12 to 1, whereas the three degree glidepath is approximately 19 to 1, so you will need to carry power if you are to remain on the glidepath.
    Glidepath and glideslope are often used interchangeably.  In my writing, I generally use glideslope when discussing an ILS as the electronic glideslope uses a “glideslope receiver” and follows a signal in space.  I generally use the term glidepath to describe the path of the aircraft or the vertical guidance provided by a WAAS GPS that calculates a glidepath, but where there is no physical glideslope signal.  Regardless, a reader will understand what you mean.

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  2. A on Aug 11, 2011

    What is the glide ratio of a cessna  1960 182c?

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