A pilot who requests flight following (VFRtraffic advisories) or who flies IFR on a cross-country can expect to talk to a number ofATC controllers. Each controller works a sector of airspace, and as a pilot approaches theborder between sectors, the controller instructs him (or her) to contact the nextcontroller on the appropriate frequency.
Complying with this instruction is simple. First acknowledge the new frequency byrepeating the ATC facility name and frequency the controller gave you. For example,"Fort Worth Center on one-two-eight-point-five, Trainerzero-zero-zero-zero-zero." Then tune the new frequency and check in. "Fort WorthCenter, Trainer zero-zero-zero-zero-zero , with you at six thousand." If you’reclimbing or descending when you check in, say "Fort Worth Center, Trainerzero-zero-zero-zero-zero, leaving four-thousand for six-thousand."
Pilots may not put much thought into checking in with a controller. They learned thephraseology 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 (orear).
Validating Mode C
Verifying the altitude information your transponder sends to ATC is called Mode Cvalidation. It means the controller has to make sure the altitude he (or she) sees on hisradar display corresponds to the altitude a pilot reports when he checks in. Thecontroller handbook says the Mode C information has to be within 300 feet of the pilot’sreport to be considered valid. An error of 100 or 200 feet is not uncommon and controllersgenerally 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 onthe 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 thecontroller’s Mode C information may be up to 10 seconds old, and controllers quickly learnwhich aircraft have this problem. Strictly speaking it’s contrary to the handbook, butthey 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. Onecomputer drives all the ATC facility’s displays, so once a controller validates the datalink from transponder to radar, it’s considered to be good throughout that facility’sairspace. Once you report your altitude to one controller, technically you don’t need toreport it to any other controller at that facility. If your hand-off information names thefacility you’re talking to, e.g., Forth Worth Center, you’ll be talking to anothercontroller at the same facility. If center hands you off to approach control, you’rechecking 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 bestprocedure is to report it every time you change to a new frequency. In fact manycontrollers will ask you to "say altitude" if you don’t volunteer it, eventhough 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 wascleared to if he has not yet reached his assigned altitude. For example, you depart froman airport served by approach control and request 9,000. Approach control only providesservice to 7,000, so that’s the maximum altitude the controller will authorize beforehanding you off to a center controller, who will clear you to 9,000 feet. Approach handsyou off to center, you acknowledge the facility and frequency, and you check in -"Kansas City Center, Trainer zero-zero-zero-zero-zero leavingfour-thousand-two-hundred."
The center controller responds with, "Trainer zero-zero-zero-zero-zero, KansasCity Center, roger. Say assigned altitude?" You verify that you’re cleared to 7,000feet. 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 assignedaltitude?"
Let’s look at this from the controller’s point of view. Yes, the center controllerknows approach should have cleared your aircraft to 7,000 feet, but what if approachcleared you to 5,000 feet because of traffic at 6,000 feet? (Controllers at one facilitydon’t always know about internal traffic within other facilities.)
Sure, approach control should not hand you off to center until it has resolved alltraffic conflicts and assigned the correct altitude. But what if you’re cleared to 5,000feet and by mistake take a frequency change meant for another aircraft? (It happens moreoften than you might think.) If the center controller assumes you’re cleared to 7,000 feetalready, and immediately clears you to 9,000 feet, he might be responsible for a loss ofseparation if the traffic at 6,000 feet is still a factor. Controllers seldom assume. Theyare 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’sreached his assigned altitude. The controller will see the Mode C display and know theaircraft has leveled off.
Leaving an assigned altitude is another matter. Let’s say you’re cruising at 7,000 feetand are approaching your destination. For whatever reason, you mistakenly believe ATC hascleared you to descend to 3,000 feet. You start down without reporting your descent from7,000 feet, not knowing another aircraft is below you at 6,000 feet. The controller won’tknow 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 aircraftbelow lose the required IFR separation. Had you reported "Trainerzero-zero-zero-zero-zero, leaving seven-thousand for three-thousand" as you startedyour 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 toclimb to 9,000 feet but you understood 7,000 feet, leveling off early might just as easilycause a problem. But there is a difference.
When an aircraft climbs or descends the controller mentally blocks all the altitudesfrom the last known up or down to the assigned altitude and doesn’t clear a nearbyaircraft to fly at any altitude in "your" block of airspace. An aircraft thatlevels off too soon at the wrong altitude almost never causes a problem because thecontroller 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 (becausehe judges that the first aircraft’s climb/descent rate will get it through the altitude inquestion before the two airplanes get too close), he gives himself enough room to changethe game plan if the first aircraft levels off too soon. In this situation, you can besure that the controller is monitoring the Mode C read-out.
Mistakenly leaving an assigned altitude is different. The controller has no reason tosuspect 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. Ifyou start a descent by mistake, and no other aircraft are below you, the controller mayclear you to the correct altitude, admonish you to pay better attention, and generallythat 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, thecontroller naturally can’t use Mode C read-out. In this case your check-in rules aredifferent. First, you must report your current altitude to the controller every time youchange 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 secondcontroller should know it, but making altitude reports safeguards against human error. Ifthe controller thinks you’re at one altitude but you’re actually at another, you’re indanger! 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 acontroller and say, "leaving two-thousand-five-hundred for seven-thousand," thecontroller must block all altitudes from 2,500 to 7,000 feet and not let any other trafficin. When you report "Trainer zero-zero-zero-zero-zero leveling atseven-thousand," the controller knows that all the other altitudes – except for 7,000feet – are available again. If you don’t report reaching your assigned altitude, thecontroller 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 allthe 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 whenleaving a certain altitude during a climb, descent, or when reaching an altitude. Thereare many possible reasons for this request, but most of them are readily apparent to apilot. 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. Yourairplane doesn’t have an exciting climb rate, and 80 miles ahead of you there’s trafficgoing in the opposite direction at 10,000 feet. The controller is concerned that you mayreach 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 makesure you are level in time. If he’s working a number of other airplanes in his sector, hewon’t have time to monitor your Mode C safely. Instead of 11,000, he may clear you to9,000 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 thecontroller to concentrate on his other traffic. When you report leaving 8,000 feet, he canreevaluate your climb against the other traffic. In air traffic control, it’s much easierto assess a situation that will take place in the next five minutes than it is an eventthat will occur in 15 minutes.
With only 3,000 feet of climb remaining instead of 7,000, it’s easier for thecontroller to determine whether it’s safe to clear you to 11,000 feet or hold you at 9,000feet until the opposing traffic is no longer a factor. The 8,000-foot "wake-upcall" 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.
When level at an assigned altitude, many pilots say "with you at…" whenthey check in with a new controller. It makes no difference if you report "Trainerzero-zero-zero-zero-zero with you at six-thousand" or "Trainerzero-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 thefrequency is busy, every little bit helps. Both reports are widely used and areacceptable.
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 suggeststhey contact the next ATC facility on a given frequency. This often happens when a VFRpilot, who isn’t yet familiar with how the ATC system works, makes his initial call forflight following.
The controller may be confused for a moment as he scans his radar targets, flightprogress 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 justheard.
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." Thecontroller will immediately know you’re a new flight that needs to be identified andprocessed. Save "with you…" for subsequent reports, when you check in afterone 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 HoustonApproach 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 haveto start the identification process all over again.