When ATC hands you off from one controller to the next, you might not give much thought to the radio procedures involved. But as Flight Training magazine's Robert I. Snow explains, there's more to it than meets the eye (or ear).
May 23, 1998
|About Flight Training ...
Flight Training is the preeminent monthly
publication for student pilots and flight instructors.
An annual subscription costs just $21.95 (higher if mailed outside the U.S.), with
longer-term subscriptions available at significant savings. You can subscribe online and help support continued free access to
AVweb and AVflash in the process.
Rember, a good pilot is always learning!
A pilot who requests flight following (VFR
traffic advisories) or who flies IFR on a cross-country can expect to talk to a number of
ATC controllers. Each controller works a sector of airspace, and as a pilot approaches the
border between sectors, the controller instructs him (or her) to contact the next
controller on the appropriate frequency.
Complying with this instruction is simple. First acknowledge the new frequency by
repeating the ATC facility name and frequency the controller gave you. For example,
"Fort Worth Center on one-two-eight-point-five, Trainer
zero-zero-zero-zero-zero." Then tune the new frequency and check in. "Fort Worth
Center, Trainer zero-zero-zero-zero-zero , with you at six thousand." If you're
climbing or descending when you check in, say "Fort Worth Center, Trainer
zero-zero-zero-zero-zero, leaving four-thousand for six-thousand."
Pilots may not put much thought into checking in with a controller. They learned the
phraseology by listening to others during their training, and generally this works.
Checking in with ATC isn't very complicated, but there's more to it than meets the eye (or
Validating Mode C
Verifying the altitude information your transponder sends to ATC is called Mode C
validation. It means the controller has to make sure the altitude he (or she) sees on his
radar display corresponds to the altitude a pilot reports when he checks in. The
controller handbook says the Mode C information has to be within 300 feet of the pilot's
report to be considered valid. An error of 100 or 200 feet is not uncommon and controllers
generally ignore it.
In some cases, a high-performance aircraft, such as a military jet or hot business jet,
can climb at very high rates, and Mode C can't keep up. This means the Mode C readout on
the controller's display may lag behind the pilot's reported altitude by hundreds of feet.
It's especially true with the slower turning long-range center radar. In some cases the
controller's Mode C information may be up to 10 seconds old, and controllers quickly learn
which aircraft have this problem. Strictly speaking it's contrary to the handbook, but
they consider the Mode C valid if it's anywhere within reason.
A controller must validate Mode C at least once in each facility's airspace. One
computer drives all the ATC facility's displays, so once a controller validates the data
link from transponder to radar, it's considered to be good throughout that facility's
airspace. Once you report your altitude to one controller, technically you don't need to
report it to any other controller at that facility. If your hand-off information names the
facility you're talking to, e.g., Forth Worth Center, you'll be talking to another
controller at the same facility. If center hands you off to approach control, you're
checking in with a new facility and should report your altitude.
The only exception is if you have not been in the facility's airspace continuously.
When ATC hands you off from center to approach control and then back to the same center,
the controller must re-validate your Mode C when you reenter center's airspace.
Rather than memorize when you should report your altitude, the easiest and best
procedure is to report it every time you change to a new frequency. In fact many
controllers will ask you to "say altitude" if you don't volunteer it, even
though ATC rules do not require it.
Stating the Assigned Altitude
Whenever a pilot checks in on a new frequency, he should report the altitude he was
cleared to if he has not yet reached his assigned altitude. For example, you depart from
an airport served by approach control and request 9,000. Approach control only provides
service to 7,000, so that's the maximum altitude the controller will authorize before
handing you off to a center controller, who will clear you to 9,000 feet. Approach hands
you off to center, you acknowledge the facility and frequency, and you check in -
"Kansas City Center, Trainer zero-zero-zero-zero-zero leaving
The center controller responds with, "Trainer zero-zero-zero-zero-zero, Kansas
City Center, roger. Say assigned altitude?" You verify that you're cleared to 7,000
feet. In the next breath, the center controller clears you to 9,000 feet, and you wonder,
"If you were going to clear me to 9,000 feet anyway, why did you ask for my assigned
Let's look at this from the controller's point of view. Yes, the center controller
knows approach should have cleared your aircraft to 7,000 feet, but what if approach
cleared you to 5,000 feet because of traffic at 6,000 feet? (Controllers at one facility
don't always know about internal traffic within other facilities.)
Sure, approach control should not hand you off to center until it has resolved all
traffic conflicts and assigned the correct altitude. But what if you're cleared to 5,000
feet and by mistake take a frequency change meant for another aircraft? (It happens more
often than you might think.) If the center controller assumes you're cleared to 7,000 feet
already, and immediately clears you to 9,000 feet, he might be responsible for a loss of
separation if the traffic at 6,000 feet is still a factor. Controllers seldom assume. They
are in the business of making sure.
Reaching or Leaving an Altitude
Once ATC validates a pilot's Mode C, the pilot doesn't need to tell the controller he's
reached his assigned altitude. The controller will see the Mode C display and know the
aircraft has leveled off.
Leaving an assigned altitude is another matter. Let's say you're cruising at 7,000 feet
and are approaching your destination. For whatever reason, you mistakenly believe ATC has
cleared you to descend to 3,000 feet. You start down without reporting your descent from
7,000 feet, not knowing another aircraft is below you at 6,000 feet. The controller won't
know that you've started down until he sees the descent in your Mode C read-out.
By this time it's too late for the controller to do anything. You and the aircraft
below lose the required IFR separation. Had you reported "Trainer
zero-zero-zero-zero-zero, leaving seven-thousand for three-thousand" as you started
your descent, the controller would have had a chance to stop and correct the situation.
At first glance it seems the situation is the same in the climb. If ATC clears you to
climb to 9,000 feet but you understood 7,000 feet, leveling off early might just as easily
cause a problem. But there is a difference.
When an aircraft climbs or descends the controller mentally blocks all the altitudes
from the last known up or down to the assigned altitude and doesn't clear a nearby
aircraft to fly at any altitude in "your" block of airspace. An aircraft that
levels off too soon at the wrong altitude almost never causes a problem because the
controller won't assign that altitude to another aircraft until he's sure it is clear.
Even if he assigns the altitude to another aircraft that is some distance away (because
he judges that the first aircraft's climb/descent rate will get it through the altitude in
question before the two airplanes get too close), he gives himself enough room to change
the game plan if the first aircraft levels off too soon. In this situation, you can be
sure that the controller is monitoring the Mode C read-out.
Mistakenly leaving an assigned altitude is different. The controller has no reason to
suspect you're about to descend, and he may be running other traffic directly below you.
And, as stated earlier, he probably will not notice your Mode C until it is too late. If
you start a descent by mistake, and no other aircraft are below you, the controller may
clear you to the correct altitude, admonish you to pay better attention, and generally
that will be all. If there is another aircraft below and standard ATC separation is lost,
you stand a good chance of facing an enforcement action.
No Altitude Reporting
If your aircraft doesn't have Mode C, or you fly outside of radar contact, the
controller naturally can't use Mode C read-out. In this case your check-in rules are
different. First, you must report your current altitude to the controller every time you
change ATC frequencies. If you don't, the controller will ask for your altitude.
A controller coordinates your assigned altitude with the next controller, so the second
controller should know it, but making altitude reports safeguards against human error. If
the controller thinks you're at one altitude but you're actually at another, you're in
danger! Reporting your altitude to every controller you talk to is good insurance.
Next, you must tell the controller when you reach an altitude. If you check in with a
controller and say, "leaving two-thousand-five-hundred for seven-thousand," the
controller must block all altitudes from 2,500 to 7,000 feet and not let any other traffic
in. When you report "Trainer zero-zero-zero-zero-zero leveling at
seven-thousand," the controller knows that all the other altitudes - except for 7,000
feet - are available again. If you don't report reaching your assigned altitude, the
controller will eventually ask you. But if you know he isn't getting Mode C information,
you can help him - and other pilots - by making the report without being asked. For all
the same reasons earlier, you must always report leaving an assigned altitude.
Even when he's validated your Mode C, a controller sometimes may ask you to report when
leaving a certain altitude during a climb, descent, or when reaching an altitude. There
are many possible reasons for this request, but most of them are readily apparent to a
pilot. One example is the "wake-up call."
Let's say you're cruising at 4,000 feet and request a climb to 11,000 feet. Your
airplane doesn't have an exciting climb rate, and 80 miles ahead of you there's traffic
going in the opposite direction at 10,000 feet. The controller is concerned that you may
reach 11,000 feet before you merge with the oncoming traffic.
If he clears you to 11,000 feet, he'll have to carefully monitor your Mode C to make
sure you are level in time. If he's working a number of other airplanes in his sector, he
won't have time to monitor your Mode C safely. Instead of 11,000, he may clear you to
9,000 feet - and ask you to report leaving 8,000 feet.
The 8,000-foot report is the wake-up call. Clearing you to 9,000 feet frees the
controller to concentrate on his other traffic. When you report leaving 8,000 feet, he can
reevaluate your climb against the other traffic. In air traffic control, it's much easier
to assess a situation that will take place in the next five minutes than it is an event
that will occur in 15 minutes.
With only 3,000 feet of climb remaining instead of 7,000, it's easier for the
controller to determine whether it's safe to clear you to 11,000 feet or hold you at 9,000
feet until the opposing traffic is no longer a factor. The 8,000-foot "wake-up
call" brought his attention back to you in time to keep you from having to level off.
A Word About "With You"
The information presented here takes some of the mystery out
of checking in with ATC and making altitude reports.
State your current altitude every time you check on to a new
If you're climbing or descending, state your assigned altitude
the altitude you're climing or descending to.
Report reaching an altitude when you're not in radar contract
or not equipped with Mode C.
Make any altitude reports the controller requests. His reason
may not be obvious, but it exists.
Always, always report leaving an altitude.
Use the words "with you..." only when you have been
previously identified and are already getting VFR or IFR ATC service.
When level at an assigned altitude, many pilots say "with you at..." when
they check in with a new controller. It makes no difference if you report "Trainer
zero-zero-zero-zero-zero with you at six-thousand" or "Trainer
zero-zero-zero-zero-zero at six-thousand." Personally, I prefer "at"
because it's shorter. The extra two syllables may not seem significant, but when the
frequency is busy, every little bit helps. Both reports are widely used and are
Occasionally, pilots use "with you" when they make their initial call to ATC,
or when a controller tells them to squawk 1200 (VFR, no radar service) and then suggests
they contact the next ATC facility on a given frequency. This often happens when a VFR
pilot, who isn't yet familiar with how the ATC system works, makes his initial call for
The controller may be confused for a moment as he scans his radar targets, flight
progress strips (strips of paper with information on aircraft already within the system),
or computer read-outs trying in vain to find information about the call sign he just
When you make your first call to ATC, say something such as "Boston Center,
Trainer zero-zero-zero-zero-zero, over Madison VOR, request flight following." The
controller will immediately know you're a new flight that needs to be identified and
processed. Save "with you..." for subsequent reports, when you check in after
one controller hands you off to another.
Also, if you're receiving flight following from one facility and the controller says
"Radar service terminated, squawk twelve-hundred, suggest you contact Houston
Approach on one-two-four-point-six" - this is not a hand-off to the next facility.
Chances are the Houston approach controller doesn't know anything about you. You will have
to start the identification process all over again.