made sense to pick Runway 6. The weather
was good enough for a visual approach, but
I always dial in the best approach guidance
available for the runway, and that is an LPV
if it’s there.
I selected the LPV to Runway 6 from the
list of procedures in the Garmin GNS 530W
and GNS 430W in my airplane and opted
for vectors to final. The letters LPV
appeared on the nav display and the
530/430. A magenta line on the G600 flat
glass nav display showed the “localizer”
path to the runway, and a magenta “G” also
appeared on the glideslope indicator. If I
had been flying an ILS, the needles would
have been green to show it is a “raw” navigation signal, not computed guidance
coming from the navigators.
Just as I intercepted the LPV “localizer”
course a message popped up on the navigators saying the approach had been
downgraded from an LPV to an LNAV (
lateral navigation only). The display reminded
me to fly to the LNAV minimums only, not
to the LPV decision height. Since the
glideslope guidance disappeared, there was
nothing to follow to decision height. The
magenta “G” and the entire glideslope scale
vanished on my primary flight display
(PFD) so there could be no mistake that I
was receiving left-right guidance only.
What happened? Where did the LPV
guidance go? I have been flying with the
WAAS navigators for several years and have
flown down hundreds of LPV courses and
never seen this before.
I consulted with Garmin’s GPS and airspace expert Bill Stone and learned that I
had encountered an extremely rare situation of unacceptable geometry from the
GPS satellites overhead. In order to triangulate an accurate position the navigator
needs to receive signals from satellites
spaced fairly widely apart so that their signals don’t line up. When satellite signals fall
close to being inline there is nothing to “
triangulate” because the signals do not form
much of a triangle.
The rules require a very high degree of
monitoring for any navigator approved for
LPV guidance, as they should since we follow an LPV signal to within a couple
hundred feet of the ground. Obviously,
accuracy is crucial, but so is redundancy.
The systems need enough satellite signals
from enough different angles to confirm
position so that a single, or even a couple of,
individual satellite signals cannot corrupt
the solution.
On an LPV approach the navigators perform signal quality and satellite geometry
analysis when you first select the approach
to be sure everything is in order. A final
analysis of the entire GPS signal and geometry status is performed just as you
intercept the final course. If every GPS
signal condition necessary to confirm the
reliability of the nav guidance is not met,
the system ends the LPV guidance and
continues with LNAV only.
LPV is the future of
our navigation system,
and it must exceed the
reliability and precision
of the ILS it is replacing.
The LNAV guidance remaining is very
precise and will guide you to the runway
centerline to within a very few meters. But
there is no vertical guidance so you must
step down to the minimum descent altitude (MDA) as you would on any
non-precision approach and hope to see
the runway from that height.
Bill tells me that the lack of necessary
signal geometry for an LPV is very rare. It
must be because I haven’t even heard of
another pilot who has had the experience.
But the switch to LNAV is out of an abundance of caution. The reason the navigators
perform the last LPV check as you intercept
is because at that point the system can
assess your groundspeed and distance to
the runway and know how long it will take
you to complete that approach. It is that
“time window” that the navigators are
looking at to be sure nothing in the necessary GPS signals moves out of tolerance
during the final approach.
LPV is the future of our navigation system, and it must exceed the reliability and
precision of the ILS it is replacing in the
coming years. And the GPS navigators
were smart enough to know that there was
at least a tiny doubt about the reliable
accuracy of the guidance to Runway 6 for
that brief period, and that’s something an
ILS can never do because an ILS doesn’t
have a brain.
SUPERSONIC GULFSTREAM?
When Gulfstream technical boss Pres
Henne was updating the media on
development of the company’s new super-
long-range and extremely fast G650 at the
NBAA convention in October, he said the
airplane had accelerated to Mach 0.995 in
dive tests. And Pres did add that it flew
“maybe a little faster.”
Certification tests for jets require a
dive test to a Mach that is at least 0.07
faster than the maximum Mach operating
(MMO) speed desired. Since the MMO
certification goal for the G650 is Mach
0.925, the M-dive test had to come ever so
close to the sound barrier.
At the altitudes where the dive test was
being conducted 0.005 Mach equals about
3 knots of true airspeed. I know the
Gulfstream test pilots are very good, but to
believe they could dive to Mach 0.995 and
then slow down before going 3 knots faster
and through the sound barrier is a stretch.
Did the G650 go supersonic? Pres won’t
say, and he only confirms the G650 satisfied the certification requirement to dive
to Mach 0.995.
The same question arose when Cessna
was certifying the Citation X with an MMO
of Mach 0.92. Its test dive maximum speed
needed to be at least Mach 0.99, and Cessna
was coy about going supersonic, too.
Come on, guys. We know both airplanes
had to be flying at least at transonic speed
briefly. But the FAA rules restricting supersonic flight are strict, so nobody is talking.
Who knows? Maybe we will all live long
enough to see a business jet at supersonic
speed in level flight after all.
J. Mac McClellan, EAA 747337, has been a pilot for
more than 40 years, holds an ATP certificate, and owns a
Beechcraft Baron. To contact Mac, e-mail mac@eaa.org.
