Descending prior to the VDP?

It's my understanding that if an instrument approach includes a VDP, you
should not descend below the MDA prior to reaching this point. If
you're not equipped to identify the VDP you should fly the approach as
if no VDP is provided.

I've been told you're not _authorized_ to descend prior the VDP but that
makes little sense. If you fly the approach without being able to
identify the VDP you may well descend prior to the VDP without even
realizing it and that would still be acceptable.

The reason that VDPs were developed is (as always) because of an accident. A
flight crew mistakenly identified the lights of a carnival with the runway
environment and descended early...into terrain. There is no excuse, no
loophole, for descending early....the consequences might be tragic. If you
fly the approach as if no VDP is provided, you will maintain the MDA until
you either pick up the runway visually or reach the MAP. In any case, in
murky conditions pilots should stay about 50 feet above touchdown zone
elevation until they see the green lights pass beneath the aircraft
(Gardner's Rule of Longevity).

Bob Gardner


Bob Gardner

"MH" wrote in message
...
It's my understanding that if an instrument approach includes a VDP, you
should not descend below the MDA prior to reaching this point. If you're
not equipped to identify the VDP you should fly the approach as if no VDP
is provided.

I've been told you're not _authorized_ to descend prior the VDP but that
makes little sense. If you fly the approach without being able to identify
the VDP you may well descend prior to the VDP without even realizing it
and that would still be acceptable.

"Bob Gardner" wrote in message ...
The reason that VDPs were developed is (as always) because of an accident. A flight crew mistakenly
identified the lights of a carnival with the runway environment and descended early...into terrain. There is
no excuse, no loophole, for descending early....the consequences might be tragic. If you fly the approach as
if no VDP is provided, you will maintain the MDA until you either pick up the runway visually or reach the
MAP. In any case, in murky conditions pilots should stay about 50 feet above touchdown zone elevation until
they see the green lights pass beneath the aircraft (Gardner's Rule of Longevity).


Excellent article about that currently at avweb...

http://www.avweb.com/news/columns/188394-1.html

Cheers,
John Clonts
Temple, Texas
N7NZ

"Bob Gardner" wrote in message
...

In any case, in murky conditions pilots should stay about 50 feet above
touchdown zone elevation until they see the green lights pass beneath the
aircraft (Gardner's Rule of Longevity).


Bob, I teach that same rule. It is like a second MDA, a Personal Minimums
MDA.

"Bob Gardner" wrote in message If you
fly the approach as if no VDP is provided, you will maintain the MDA until
you either pick up the runway visually or reach the MAP.

If I pass the VDP without seeing the runway environment, I know that I won't
likely make the runway from the MAP. After passing the VDP, I start
preparing for the missed approach.

D.

Sure, but it doesn't answer my question which more a regulatory one. Are
you "not supposed to" or "not authorized to"?

Capt.Doug wrote:
"Bob Gardner" wrote in message If you
fly the approach as if no VDP is provided, you will maintain the MDA until
you either pick up the runway visually or reach the MAP.


If I pass the VDP without seeing the runway environment, I know that I won't
likely make the runway from the MAP. After passing the VDP, I start
preparing for the missed approach.

D.

"MH" wrote in message Sure, but it doesn't answer my question which
more a regulatory one. Are
you "not supposed to" or "not authorized to"?

If you can maintain suitable cloud clearance and maintain suitable
visibility, you can descend prior to VDP legally. This practice is frowned
upon since descending before VDP puts you into unprotected airspace. You may
have visibility which is sufficient to keep the runway in sight, but you may
not see the antenna farm that the approach was designed to keep you over.

D.

The FAR require you to have the runway environment in sight before
descending from the MDA. Reaching VDP is not an authorization to
descend below MDA. You must have the runway environment in sight. The
purpose of VDP is to help you make a comfortable glide to the runway.
You can treat the VDP as the point beyond which you cannot make a
normal landing to the runway. If you pass the VDP without runway in
sight, start getting ready for the missed approach. Then begin the
missed approach after passing the MAP.



MH wrote in message ...
Sure, but it doesn't answer my question which more a regulatory one. Are
you "not supposed to" or "not authorized to"?

Capt.Doug wrote:
"Bob Gardner" wrote in message If you
fly the approach as if no VDP is provided, you will maintain the MDA until
you either pick up the runway visually or reach the MAP.


If I pass the VDP without seeing the runway environment, I know that I won't
likely make the runway from the MAP. After passing the VDP, I start
preparing for the missed approach.

D.

This is my understanding as well, the VDP is the optimal point at which
to start the descent to the runway if 1. the visibility requirements are
met, 2. the aircraft is continously in a position where it can land
using normal maneuvering and descent rates, and 3. has the runway
environment in sight.

The longer you fly after passing the VDP, the harder it becomes to make
the landing. This does not mean that you're not ALLOWED to descend prior
to the VDP, and in fact you very well may be if for example the VDP is
identified through a dme distance, and the plane doesn't have a dme
onboard. In that case, you level off at the MDA and upon establishing
visual contact with the runway, you descend as you deem appropriate and
land. This MAY result in you descending below the MDA prior to the VDP,
but you wouldn't know it. If the approach is designed correctly then the
point where it feels natural to start the descent to the runway should
more or less coincide with the VDP anyway so it shouldn't be that big of
a problem.