Sinkholes are
certainly not uncommon to Gauteng. In fact, the eastern parts of
Rietvlei Dam Reserve are riddled with Karst activity and the area
is well known for its sinkholes. These sinkholes, which range in
size and depth, are mostly hidden from public view because they are
situated on privately controlled property. Larger sinkholes are
easily visible on satellite and aerial photos. On Google Earth
™ at S 25° 52.041' E 028° 18.806' there is an interesting
example that is situated just north of the Lion Enclosure to the
east of the R50 Delmas road.
This cache hunt
will take you to the scene of a much smaller sinkhole that is
easily accessible to the public and which can be reached by using a
standard vehicle. The cache is hidden close to Rietvlei View, to
the east of the R50 Delmas road. There are another 3 caches in the
vicinity, namely:
GC16HPZ - A Model View by Cabey;
GC25JMK - MF2010 Lions Inside by Dakardrix ; and
GC167YA - Mooikloof Heights Windmill by Cabey.
*click on the on
the above hyperlinks to view the respective cache listings
;
On a cool winter morning, a
microlight pilot and his observer were flying in the Rietvlei Dam
vicinity looking at game. They flew low and slow in order to
maximise the probability of viewing game. During the flight they
observed what they thought was a very deep sinkhole so they flew
closer to obtain a better view.
They were surprised to see
that it was indeed a sinkhole measuring approximately 12 metres
diameter and of considerable depth that they could not estimate
accurately. After circling the sinkhole a few times, they flew
directly overhead and made a final fly-by on a compass heading of
119°. The pilot flew at a steady speed of 45 kts according to his
airspeed indicator, and he maintained a steady heading and
altitude.
Exactly 8.6 seconds after
overflying the sinkhole, the observer noticed two suspicious
looking persons directly below them on the ground. The observer was
convinced that these persons where hiding something amongst the
rocks. The pilot had to make a precautionary turn to the left to
avoid flying into the high-tension power lines and during this
turn, the observer saw these persons hastily fleeing the scene on
off-road motorcycles.
After landing back at the
airfield, the pilot and observer performed the usual after-flight
inspection of the aircraft and closed up the hangar before going to
have a drink at the clubhouse. They discussed their exciting
experience and decided to return to the scene of their adventure to
see more from the ground. From their consultation of some maps and
the meteorological data for the day the gathered the following
information.
- Sinkhole position : S 25°
53.236' E 028° 19.722'
- Closest safe parking : S
25° 53.220' E 028° 19.761'
- Wind Direction and Speed :
143/22
- Pressure Altitude :
4,904'
- Barometric Pressure : 1016
hPa
- Calibrated Airspeed : 45
kts
- Compass Heading :
119°
- Compass Deviation : 2°
under read
- Magnetic Variation :
17°W
- Time from overhead sinkhole
until overhead the spot : 8.6 sec
Can you assist the crew to
find whatever the alleged villains were hiding between the
rocks?
SOLVING METHODOLOGY
Traditionally, in air navigation, displacement of position caused
by wind was calculated by applied mathematics to solve for the
vectors of a wind triangle. Dead Reckoning (DR), which is a term
derived from "Deduced Reckoning" was used to calculate aircraft
position. It is rarely used in the traditional form for air
navigation, but it survives in the form of Inertial Navigation
Systems (INS), which are nearly universal on more advanced
aircraft. The INS is used in combination with other navigation
aids, such as GPS, in order to provide reliable navigation
capability under virtually any conditions, with or without external
navigation references.
It is important for the pilot to be trained the basics of DR in
case of a system failure, or if the aircraft is only equipped with
less advanced systems.
After solving for True Airspeed (TAS), a variety of methods can be
used to solve the vector portion of the puzzle.
The most practical methods include the following:
- Traditional vectors on
paper;
- Navigation formula and a
programmable computer (for example a
HP-41C™);
- Dedicated Aviation Circular
Slide Rule/Computer (such as a Jeppesen™ or Aristo™
);
- Dedicated handheld electronic
aviation computer (such as a Pathfinder CX-1™);
or
- Internet based "On-line
computation"
USEFUL LINKS
Click here to read about Dead Reckoning on
Wikipedia.
Click here to read about the Wind Triangle applicable
to air navigation.
Click here to download the Cache Owner's online
solving assistant that runs in Excel. The file is "Read Only" and
can be saved on local drive with a new filename, or it can be used
online without saving any data.
CHECK YOUR
SOLUTION
HALL OF FAME
- FTF Honours: pannie&medusae
- Second to Find: Jors & Sonsoekster (Joint
find);
- Fourth to Find:Alien3Inc & Cabey
(Joint
find).