Aviation Topic of the Week
By Michael Oxner,
August 17, 2003
This week's topic:
Variety Q & A #2
I've run a little short on time to produce the planned segment for this
week, so I ran with some short questions I received. Many of these questions
came from the same reader, actually, and I'd like to express thanks for sending
them in. Perhaps these will spark some new questions?
Altimeter Setting and Mode C Readouts
Cold Weather Altimeter Errors
Radar Coverage and Weather Information
Factors Affecting Radar Range
ATC and Multiple Frequencies
Readbacks of IFR Clearances with SIDs
Altimeter Setting and Mode C Readouts
The first question related to Mode C readouts and how a pilot setting
his altimeter incorrectly would affect his Mode C readout on the radar screen.
We'll talk about altimeters first, and how the setting affects the instrument's
accuracy itself.
The old rule applies: If it's set too low, it will read too low. Here's
the set up. Local sea level pressure is 30.20, but you've set your altimeter
to 30.00. You're cruising at 3,000 feet ASL, and your altimeter reads 3,000
feet. You'll actually be cruising at 3,200 feet. The simplified equation
is 1,000 feet per inch of Mercury, and your error of 0.20 inches translates
to 200 feet of error. Since the altimeter is set too low, it will read an
altitude that is lower than the true altitude, in this case by 200 feet.
You're actually higher than what you're indicating. This is the lesser of
two evils. If the station pressure is 28.90 and you set 29.90, you're going
to read an altimeter that is showing 1,000 feet higher than you really are.
You'll think you're leveling off at a safe altitude on the approach, only
to find out that you're instrument is set in such a way that you'll be flying
1,000 feet too low. Hence the requirement for ATC to state the altimeter
twice when it's below 29.00, so that pilots will be reminded that the setting
is abnormally low. Typical barometric pressure at seal level ranges between
29.00 and 31.00.
On to Mode C. Mode C altimeters always report altitude based on 29.92,
the standard setting. ATC radars receive the altitude reported by your
aircraft's transponder, and then correct it for local pressure before displaying
it to the controller. So, in the above example, if local pressure were 30.12,
and you're altimeter is correctly set to 30.12, your Mode C is reporting
2,800, but ATC's radar, assuming his altimeter data is current, will correct
that to 3,000 again. If you inadvertently set your altimeter to 29.92, 20
points too low, and fly at an indicated altitude of 3,000 feet, you'll actually
be cruising 200 feet higher than you show. You'll now be cruising at 3,200
feet. Your Mode C will transmit the altitude based on 29.92, so it will send
3,000 feet. ATC's radar will correct that based on current pressure of 30.12,
so it will add 200 feet, displaying 3,200 feet on the screen. ATC may ask
you to verify your altitude and he should restate the current altimeter setting.
When he does this, check what you have set. You'll reset to 30.12, immediately
show 3,200 feet, and descend to 3,000.
If a pilot forgets to set his altimeter to 29.92 above transition altitude
(this is FL180 almost everywhere in Canada and the US), the altitude readout
will vary as well. The pilot will be reading an even flight level on his
altimeter, but the altitude reported by the transponder will be based on 29.92,
and will be different. ATC radars will not correct this, and the Mode C reading
will show the aircraft's actual flight level, not what the pilot is seeing
on his altimeter. As above, the difference will be based on the difference
between 29.92 and the altimeter setting still on the altimeter.
Cold Weather Altimeter Errors
Our altimeters in our aircraft are pretty good instruments, but they
are prone to errors of many kinds. The only time they will show true altitude
is in Standard Atmospheric Conditions. These occur when temperature is
15° Celsius, sea level pressure is 29.92 inches of Hg, the decrease
in temperature with altitude is 1.98C° per 1,000 feet, and so on. Anything
outside of this set of conditions will lead to errors, most of them small.
When the temperatures are very cold, the accuracy of the altimeter drops.
In the General section of the Canada Air Pilot, there is a
chart to be used when temperatures are cold, allowing pilots to determine,
based on altitude above the reporting station, the correction factor to be
applied to all altitudes, most importantly while on approach to an airport.
If the correction factor is 220 feet, and Decision Height is 680, you now
have to make the go/no-go decision on landing by the time you reach (680+220)
900 feet ASL as indicated on the altimeter..
These altitude corrections should be applied whenever the aircraft is
operated with marginal ground separation, including all segments of instrument
approach procedures (IAP). The exception is when you're on a radar vector
from ATC. When radar vectoring altitudes are established, weather records
for the station in question are checked for a given period in history. This
period consists of many years, but I'm not 100% sure of the number. The lowest
temperature that can be reasonably expected historically is used to make
the correction to be added to the lowest altitude found that covers a number
of factors. The base of controlled airspace, terrain, obstructions, etc.
The necessary correction is then added to the "floor" determined considering
all of these factors. The reason this is done is that ATC is responsible
for terrain and obstacle clearance when he starts vectoring your flight.
If you're being vectored for an approach, once established on the course you
were told you're being vectored to, you must then resume the corrections to
the altitudes on the approach plate.
One last note, I honestly don't know if Flight Simulator simulates these
errors. Anyone else know?
Radar Coverage and Weather Information
Offering significant weather information is part of ATC's job. It is
secondary to managing traffic, but it is integral. Safety can be an issue,
especially in certain types of weather. In Canada, NavCan's current set
up for weather information is inferior to former radar systems, largely
because of the wavelength used for ATC's primary radar (secondary radar
uses the transponder, while primary uses reflected radiation). If we see
it, we tell the pilots about it. Since airborne weather radars are actual
weather radars built for the purpose, they get better definition of intense
weather systems. I always rely on reports from aircraft before my own radar
presentation of weather. I only rely solely on my display when the aircraft
involved doesn't have a weather radar, and even then I often ask for reports
from aircraft so equipped that are near by. Because not all of NavCan's radars
have primary radar capability, many sites will not even show weather (for
that matter, an aircraft with no transponder will not show on radar either).
Factors Affecting Radar Range
Many things affect the useable range of a radar antenna. First off, ATC
radars operate at wavelengths that are line-of-site. For SSR, ATC radars
transmit interrogation pulses on 1030 MHz, and receive aircraft replies on
1090 MHz. Both of these frequencies are in the UHF band and cannot normally
bend over the horizon. I have seen many formulae attempting to approximate
the line-of-sight range, and one that isn't all that bad is the square root
of the altitude. Line-of-sight also includes terrain and other significant
obstructions, so mountains will also limit the radar coverage by raising
the "radar horizon". Another phenomon that affects radar coverage greatly
is called an "inversion". This is where the temperature rises with altitude,
instead of the normal decrease with altitude. The radar antenna can often
see over the horizon quite effectively under such circumstances. I once watched
a helicopter depart an offshore oil rig 160 miles from the closest radar
and had good, solid radar contact out of 100 feet ASL at that distance. Whatever
the case, NavCan's primary radar (PSR) is limited to an 80 NM range, with
weather presentation limited to 100 NM from the antenna. Secondary Surveillance
Radar (SSR) range is dramatically longer, since airborne equipment (transponders)
can send a signal that would far out power reflected radiation from an aircraft
beyond the 80 NM range of our primary radars. SSR is capable, if the conditions
are right, or the aircraft is high enough, of "seeing" out to about 250
NM from the antenna.
ATC and Multiple Frequencies
One question received involed the use of multiple frequencies by ATC.
This is most notable in ATC sectors that include larger areas and include
low level airspace. Because of the range limitations of VHF and UHF communications
at lower altitudes, ATC must often have more than one frequency to adequately
cover their airspace. In fact, over a midnight shift, the Moncton low level
specialty where I work monitors 17 frequencies, not including the UHFs
we have. And we're the smallest FIR in Canada right now. In the real world,
we have touch-screen panels which have a little box representing the status
of each transceiver. The appropriate box lights up green showing us which
one is receiving a signal. If more than one aircraft transmits at a time,
this can be confusing, as often happens when monitoring so many lines at
once. For this reason, they have been attempting to get a piece of equipment
working which relays aircraft transmissions from one receiver over all the
others the controller is monitoring by repeating these received transmissions
over the ATC transmitters. This way, pilots on different frequencies can
hear others talking and know not to speak up. This has its drawbacks, too.
Pilot friends and coworkers often take ATC air time to acknowledge each other's
presence, or worse, have conversations. This will be more prevalent when
this equipment works since pilots on different frequencies will be able to
hear others. If you monitor ATC frequencies, this will be a boon to you, since
you'll hear more if you live near a transmitter. Some centers in Canada already
have this working, but not on all radios.
Many controlled airports will also experience this. Often a controller
in a slower airport will be working the unit alone, and some will choose
to transmit on the ground and tower frequencies simultaneously. This is
why you may hear taxi instructions issued on a TWR frequency and take-off
and landing clearances issued on a GND frequency.
Readbacks of IFR Clearances with SIDs
The last question to be addressed this time around includes the provision
for abbreviated readbacks of IFR clearances. When a SID is issued as part
of the ATC clearance issued prior to departure, the pilot need only acknowledge
the clearance by repeating the aircraft callsign and the transponder code
assigned. If there were any ammendments to the SID (such as altitude or
heading), these must be read back in full. ATC may request a full readback
of the IFR clearance as issued, and the pilot is required to comply, according
to the AIP, RAC 6.1, ATC Clearance.
I'm open to any feedback, corrections, additions, and other information.
My e-mail is moxner@nbnet.nb.ca.