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 ear).
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 four-thousand-two-hundred."
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 altitude?"
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 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 even-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.
Controller Requests
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"
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 acceptable.
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 flight following.
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 heard.
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.
Check-in Reminders
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 frequency.
- If you're climbing or descending, state your assigned altitude - the altitude you're climbing or descending to.
- Report reaching an altitude when you're not in radar contact 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.
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