- FAA certified VFR private pilot
(aircraft up to 12,500 lbs) + instrument rating student
- President and Board Director,
Plus One Flyers, Inc. of
San Diego, CA (900+ pilots, 90+ flight instructors and 45 aircraft at
- Flying for fun, weekend
getaways, camping, training, etc.
Cirrus SR22 N8148F
Papers and Notes
Flight Photos & Adventure
Nowhere near every flight gets
captured here, but when I take a photographer along a good
flight sometimes has a few thousand words to make it a small
How Ditch-Risk-Free is your flight to AVX on Catalina Island?
When reviewing my flight plans for Avalon airfield (AVX) on
Catalina Island about 30 miles from the California coastline,
I wondered just how well-managed the risk of ditching in the
ocean really was for the single-engine aircraft I flew.
Ditching in the ocean is not considered optimally survivable
in fixed-gear aircraft like a Cessna or Piper, where the
typically plane flips over upon impact and usually starts
sinking nose-first, quickly. Even if you are not seriously
injured or knocked senseless and able to act after the impact,
immediate egress from the aircraft should be counted in
seconds. Then there's open-water survival to deal with, not to
mention any passengers you brought, even if you are relatively
close to shore. No thanks - I'd rather land on a beach
or shoreline road any day.
Being fairly conservative, several of
my flights from San Diego's Montgomery Field (MYF) flew north
up the coastline and turned to AVX at Dana Point, crossing at
10500 MSL and returning at 11500 MSL. The 180-HP C-172N I
usually flew to AVX cruises at about 125 KTS, and has a POH
glide ratio of about 9:1. At that ratio, gliding to AVX,
having a field elevation of 1600 MSL from a cruise altitude of
10500 ft should be possible from 13 NM away. Similarly,
gliding back to the California coast shoreline and leaving 500
ft of working altitude is almost 15 NM. As such, from 10500
MSL, my flight path should not exceed 15+13 = 28 NM of open
water, with a landfall commit changeover at 15 NM from the
coast when there are no winds aloft.
Winds aloft that are primarily
east-west change the commit changeover point. The primary
concern is winds aloft that can blow a gliding aircraft north
or south of either the coastal shoreline, or AVX. Dana Point
is 35 NM and Huntington Beach is 25 NM from AVX. At 125 KTS, a
gap of 35-28 = 8 NM is a ditch-risk window of 4 minutes with
no winds aloft.
Check out my paper on GPS
Measurement of Aircraft Glide Ratio for supporting
- GPS vs. (Barometric)
Aircraft Altimeter Altitudes for VFR
During some of my summer flights in
particular, my Garmin GPSMap 296 showed altitudes hundreds of
feet higher than the panel altimeter, also observed by my
non-pilot friends in the aircraft with me.
I attributed most of the difference to
non-standard temperature (heat-related) density effects,
however the GPS unit indicated we were a few times near class
B airspace floors when the panel altimeter indicated we were
plenty low enough below it. So I hit the books and
here's what I found:
(a) Fly by the altimeter, not the
GPS, for cruise and airspace altitude selection - as long as you have a valid altimeter setting per
FAR 91.121. For now at least, the FAA expects you (and
all other pilots) to refer to the barometric altimeter in
the aircraft for your cruise and airspace altitude selection decisions,
so long as safe terrain clearance is observed.
This guarantees all planes'
altimeters in a given vicinity experience the same
conditions, and therefore the same errors, because
barometric altimeters are quite precise even though
uniformly inaccurate from local factors (ATIS/ASOS/AWOS
settings, altitude, heat, humidity). Consequently,
collision risks are mitigated because of the FAR-mandated
common reference is maintained for standard altitude
(b) Terrain avoidance with GPS is
generally better - though it depends upon which GPS
equipment you are using. Most current aviation GPS units
display 'virtual' MSL altitudes by taking the local EGM96
geoid corrections into account, and with WAAS are both
accurate and precise. (See the
discussion of GPS Aircraft Altitude Accuracy below.)
(c) Jedi flying trick: "ATC this
is not the altitude you're looking for..." - My (VFR)
GPS could even indicate we are in
an airspace altitude bust condition but if the barometric
altimeter says otherwise, we are fine so long as we have
the right setting. The mode C transponder tells ATC what
your altimeter reads and if the Kollsman window setting
complies with local ATIS/ASOS/AWOS, etc., ATC 'sees' what
your altimeter indicates.
(d) GPS WAAS with barometric
altimeters - Stanford University
published a FAA-sponsored research paper to examine
tradeoffs and confidence margins of using 1, 2, or 3 WAAS
frequencies with and without a barometer. A review of
barometric altitude computation is included.
(e) AviationBanter.com thread on
this topic - I found
this thread after I wrote up most of these two GPS
When the FARs update in the future
to refer to GPS altitudes, we will still have these issues
to content with, but in reverse, because a barometric
altimeter will be kept as a backup to the GPS or built into
it, as some GPS units already do.
- GPS Aircraft Altitude
Were those observed altitude differences
between my GPS and the barometric altimeter explained
entirely by density
effects and local weather? Back to the books and not a little Google
searching and cross-checking. Here's what I learned:
With WAAS, GPS altitude errors have
been within 2 or 3 meters (6-10 ft). But GPS altitude errors are
unrelated to altitude standard of
(a) FAA aviation charts and airways
refer to Mean Sea Level (MSL), the 19-year-(Saros
cycle)-tidal-average local sea level, also called the
orthometric reference datum.
NASA/NIMA EGM96 geoid reference model closely
approximates MSL to within a few feet in most cases (or
+66/-132 feet [Harvard]) using an
order-360 and degree-360 spherical gravimetric potential
harmonic model having over 130,000 coefficients.
As such, MSL and the EGM96 geoid
datum are close enough to be considered the same in most
Order 8260-48 RNAV Approach Construction, 1.4.10). So here I'll call the EGM96 geoid reference 'virtual' MSL, or vMSL for short.
(b) However most U.S. GPS device
calculations internally reference the more perfect and
WGS84 reference ellipsoid datum. In the San Diego/Los
Angeles area, the
geoid (vMSL) is about 105 feet below the WGS84 ellipsoid.
In other places
EGM96 and WGS84 can differ by more than 300 feet [NASA/JPL].
Source: NASA JPL
(c) Most GPS units
display vMSL altitudes by interpolating a stored table
of EGM96 geoid heights above the WGS84 ellipsoid. Garmin IFR models 430/530 and
convert their internal WGS84 altitude to vMSL using the
EGM96 geoid datum model.
(d) When I checked the default datum in
my Garmin GPSMap 296 (VFR), sure enough, it was WGS84. I
would have thought this means to find
the vMSL altitude in my area, I need to add 105 feet per (b)
above, to the
unit's displayed altitude.
However, my GPS unit appears to
display vMSL atlitudes. If my GPS displayed WGS84
altitudes, I should expect the unit to show about 427 MSL
elevation - 105 ft geoid height = 320 feet altitude on the
ground at MYF airfield. When I performed an on-field
test with WAAS correcting 11 GPS satellites,
my GPSMap 296 reported the MYF field elevation as 415 to 420
feet, +/- 8 ft, pretty much the correct MSL elevation.
(e) AviationBanter.com Thread on
this topic - I found
this thread after I wrote up most of these two GPS
I'm not certain but I'm guessing it's because VFR GPS units do not
have as extensive a geoid table or specific details for RNAV/VNAV
approaches in particular that the FAA authorizes them
only for VFR situational awareness, not as primary flight
But in an partial panel or IMC
emergency it's a good resource to have for turn/bank
information as much as for position and altitude - my GPSMap
296 has a simulated instrument panel that tracks quite well to
compass, turn and bank. But knowing a WAAS altitude is
only a few feet off is still helpful.