Aviation Topic of the Week
By Michael Oxner, October 5, 2003


Introduction
Previous Week's Topic
Following Week's Topic
Aviation in Canada Blog
Archives

This week's topic:
Airport Operations Part 2: Taxiing

Last week, we looked at Apron Movements. This week, Taxiing is the topic. This includes both sides, ATC and piloting, and includes taxiing for departure and after arrival.

Departures
    Before You Call Ground
        Obtaining IFR Clearance
    Taxi Information and Instructions
        Clearance Rules
        Taxiing On Runways
        Taxiing Across Runways
        Readbacks
            Hold Shorts
            Others
Arrivals
    Where to Clear a Runway
    When to Call Ground
ADDENDUM
ADDENDUM #2

Departures

So, you're in your airplane. Either you're at a gate on the terminal apron, or you're parked on a private apron getting ready for departure. You've completed the walk around, filed your flight plan, and obtained your briefing including NOTAMs for the departure, destination and alternate aerodromes, and the FIR(s) for which you're flying through. You're fully aware of the current weather conditions along your route of flight, you've loaded up your aircraft and you're ready for engine start. What else are you supposed to be doing? If you're IFR, you need an IFR clearance. If you're VFR, to heck with that, let's start up and run.

We'll look at the extra step of the IFR clearance and then return to the taxiing part. While most IFR clearances are obtained while the aircraft is on an apron, the IFR clearance isn't required to move on an apron, so I didn't include them in last week's topic. At most larger airports, IFR clearances can be obtained well prior to engine start in a pre-taxi, canned format. They almost always include the minimum requirements which are basically a clearance limit (usually the destination airport), a SID, a route (almost always "Flight Planned Route" or some other variant of this type of phrase), and a transponder code to "squawk". Some airports have some local procedures which include extra information, and also some places include a runway for departure with this clearance. Since I have already discussed IFR clearances and their elements, I'll leave you to the Archives for the meanings of each of the above items. Quite often, an airport served at this level has a separate Clearance Delivery frequency established. Even if DEL isn't running separately, this is likely what the airport TWR will provide upon request for IFR. If they don't provide this level of service, ATC will advise the pilot when he calls and let him know when the best time to call for clearance is, whether it's now, prior to engine start, or after start up when the aircraft is ready for taxi. Also, if DEL doesn't exist at your favorite airport, look for GND and request IFR with them.

Oops. Forgot one step. Get the current ATIS, if the airport has one. Note the letter of the ATIS. When you call for clearance, inform ATC that you have their ATIS with the identifying letter, and simply state that you're requesting IFR. They'll either give you the clearance, or they'll ask when you'll be ready and let you know when to call.

For smaller airports, especially those not served by an ATC Tower, or for which the TWR is not in operation at the moment, the IFR clearance can still be obtained prior to taxi, but the IFR ATC unit may not wish to issue it until they know you will be airborne within a fairly short period of time. The more in advance of being airborne the clearance is issued, the more likely it is the clearance will be overly restricted so ATC doesn't have to restrict other aircraft around you, since he doesn't know exactly when you'll get in the air. At such airports, it is quite often the case that ATC would rather wait until you have the engines started and are ready for taxi so they can expect you to be airborne within a few minutes of the time the clearance is issued. If ATC tells you to call back for IFR when you're ready to taxi, you might want to ask if they anticipate any delays, just to be on the safe side. You can then decide when to start engines and avoid burning extra fuel.

Taxi Information and Instructions

Now we'll assume you're engines are started and you're ready to taxi. If you were told to call back for your IFR clearance, now's the time to do it. If you're VFR, no problem, unless you're operating in a Class B or C control zone. Then you'll need a clearance, too. Once you have the clearance required to go flying, you need a clearance to go taxiing, if you're at a controlled airport. Now we get to the meat of taxiing.

People who have flown before have heard the format from Ground. The conversation goes something like:

Pilot: "Halifax Ground, Cessna Golf Oscar Oscar Foxtrot"
Ground: "Cessna Golf Oscar Oscar Foxtrot, Halifax Ground"
Pilot: "Ground, Oscar Oscar Foxtrot with you at IMP, with information Alpha, ready to taxi for one hour of circuits."
Ground: "Oscar Oscar Foxtrot, runway 06, winds 040@5, time 2026, altimeter 3025. Taxi Alpha and hold short of Runway 06"

The pilot has, in this example, called GND with his aircraft type and callsign. That's all that should be said initially, since GND may not be ready to absorb the rest. He has phones to answer, other controllers to coordinate with, etc. Just call up first to get his attention. When he answers, continue on.

The next transmission gives ground everything he needs to know about your flight. Where you are, if there's an ATIS, which letter you have so he knows you have the current information for the airport, and what you plan to do. Many airports have additional information they'd like to hear, but this comes down to local procedure in most cases. If you're on a VFR flight plan, it's a good idea to mention that, since he's likely to have some information on you already.

When GND comes back to you with instructions, it's a good idea to have a pencil and paper ready, just in case it gets complex. Our example is pretty simple, but some airports are complex and directions might be difficult to remember. Ground is supposed to give you taxi information before taxi instructions. The format is as in the example above: Runway, Wind, Time, Altimeter. Some will already be shaking their finger at me, since I actually overdid the GND's statement in the above example. Since the ATIS already has the active runway and winds announced and the pilot acknowledged the current ATIS, GND could leave out those two pieces of information. If you're IFR and you got your clearance prior to taxi, DEL/GND/TWR may have already given you this information along with acknowledging your presumably correct IFR clearance readback. Then, they can launch right into the taxi instructions when you call for taxi clearance.

So now on to the taxi instructions. Put simply, follow the directions. There is no real speed limit on taxiing, but as my flight instructor used to state, "if you're airspeed indicator needle comes alive, you're probably taxiing too fast". Airmanship should rule here. You don't want to race across the field at near rotation speeds. There is one more important transmission to come from the pilot to finish the conversation in the example above. Whether you can figure it out or not, read on.

Clearance Rules

ATC has a number of phraseologies to use for ground control. Some are to keep phraseologies standard, but others are rules as to what not to say. Often, these are borne out of incidents. For example, GND controllers are not supposed to tell vehicles to get off a runway by saying, "report clear of the runway". History has it that an operator (or more than one in separate incidents over time) misheard ATC and thought he said, "report clearing the runway". He figured he was clearing the snow off the runway, so he said, "I'm clearing the runway now". Then a landing or take-off clearance was issued and a surprise runway incursion transpired. Terms like, "vacate the runway", or "exit the runway" are now prefered, since they are less likely to be mixed up. Now after saying all of that, there aren't many snowplow drivers in VatSim. This makes GND control's job much simpler in the winter time.

ATC, when telling an aircraft to taxi, is expected to use the term "taxi" as their instruction. Not "drive", or "roll". "Proceed" has also been used, though it's more appropriate for ground vehicles. Please see the ADDENDUM below for another regulation that is very important to airport control.

Since the above example includes a taxiway that runs right up to the runway to be used, it is acceptable for the GND controller to have said, "... Taxi Alpha to runway 06" without actually using the term "hold short of runway 06". The AIP RAC 4.2.5 says, "Under no circumstances may a taxiing aircraft, whether proceeding to or from an active runway, taxi onto an active runway unless specifically authorized to do so."  It goes on to say that a pilot is supposed to taxi to the holding position for the assigned runway, and that will normally be marked on the taxiway by yellow markings across the taxiway. In the event that markings are not established, the pilot should remain at least 200 feet from the runway edge until further authorization is received. This last part is contained in the AIP RAC 4.2.6. It further states that if a pilot can't comply with either of the above mentioned, the pilot must hold at a sufficient distance to prevent causing a hazard to arriving or departing aircraft.

What if the clearance included a taxiway that crosses a runway? The same section of the AIP, RAC 4.2.5, states that GND will specifically state whether to cross or to hold short of the runway. For example, aircraft using taxiway Delta on the way to the threshold of runway 33 at Halifax must cross runway 24 to get there. Ground should be saying either, "Runway 33, winds 310@5. Taxi Delta across runway 24, hold short of runway 33", or, "Runway 33, winds 310@5, taxi Delta, hold short of runway 24". Either way, runway 24 is mentioned. The AIP also states for pilots that if no specific authorization to cross was received, the pilot shall hold short and request authorization to cross the runway. Runway incursions are a plague that must be avoided, so these rules are made, and made to held to hard and fast, not broken.

Another important job for ATC is getting those darn hold short readbacks. Making sure runway incursions don't happen is important to a GND/TWR controller's job. To this end, it has been entrenched in the regulations that anyone, aircraft or otherwise, told to "hold short" of anything, be it a taxiway, runway, or a gopher on the pavement, must readback the hold short condition. Further, if a pilot or vehicle operator forgets to read it back, ATC must chase after him for the readback. Having said the hold short, it is believed the pilot will be more conscious of the restriction when he reaches the hold point and be less likely to cross the line into the paperwork regime that follows an incursion.

Back again to the AIP, RAC 4.2.5, the same paragraph goes on about readbacks. Yes, a hold short instruction requires a readback. Note that nothing else does. CAR 602.31(1)(a) refers to the requirement for a readback of a hold short instruction, but requires nothing further. This means that if a pilot hears, "Taxi Alpha, Charlie, Golf, Hotel, across runway 27, Kilo, Whiskey 2, hold short of runway 01", the pilot is only required to read back "hold short of runway 01". The idea is to reduce the chatter on radio frequencies. If you're unsure of the route issued, if you missed something in it, or just want to confirm what you were given, it's not illegal for you to readback a taxi clearance.

Arrivals

The next most common question regarding taxiing is for arriving aircraft. There seem to be two different groups. One group says they can't get a consensus of what they are supposed to do, and other group can't believe the first group exists since "it's so obvious". I guess that's like most topics, isn't it?

Anyway, the question tackled in this subsection is primarily one of what to do after landing. The AIP RAC 4.4.4 gives us the rules, but leaves out the situation that causes a few questions to arise in some people's minds. We're told by the AIP that, unless otherwise instructed by ATC, an aircraft is to continue straight ahead on the runway and exit on the nearest taxiway. Note that statement doesn't include another runway as a place to exit the landing runway. The rule as stated in that section of the AIP continues to say that an aircraft should not exit a runway onto another runway unless directed by ATC. So cross whatever runway is there and vacate the landing runway on the next available taxiway. Since the rule states that the aircraft must taxi straight ahead, what if there is no taxiway past the runway you just crossed? What if there is only one taxiway and you passed it? Hopefully the TWR would be aware of this situation (since they should be watching, right?) and would give the authorization to backtrack if required, or to use the runway you're about to cross (traffic permitting) as an exit point. Typically the TWR will give instructions as to where he wants the aircraft to exit the runway. Further to the direction issued above by the AIP, all aircraft should, when vacating a runway, taxi to such a point where all aircraft parts are past the taxi hold lines, or at least 200 feet from the edge of the runway. The aircraft will not be considered clear of the runway until one of these conditions is met.

Normally, once an aircraft has landed, the TWR will give instructions as to when he wants the pilot to call Ground Control. Something like, "Oscar Oscar Foxtrot exit right in Charlie and contact ground 121.9 off the runway". The pilot is actually expected to listen out on TWR frequency until he's clear of the runway, and then call GND when he is far enough away, as described above. Further taxi instructions would be provided by GND at that point. There is a bit of a gray area that has mixed opinions in both the Tower and the cockpit here, too. This is when the taxiway used to vacate the runway is only a short one connected directly to the terminal apron. The situation that gives rise to the gray area is when an aircraft exits the runway on such a taxiway, and GND is too busy to talk to the aircraft right away. Since he's blocking the taxiway and the apron is right there, is he allowed to taxi into the apron without authorization rom GND? Should he sit patiently in the taxiway to get the go ahead to move that next 100 feet? I've seen controllers get mad because an aircraft chose to do it, and I've seen controllers shake their heads because the pilot came to a stop when he's only a few feet from the apron and couldn't get through. I've seen pilots say, "what's the difference?" and do it, and others who say, "you haven't got a leg to stand on if you move ahead without authorization." Unfortunately, this situation isn't clarified in the AIP for pilots and controllers. The best I can offer is this: Use good judgement, and consider the situation. Look at the traffic, consider the communication on GND that is blocking your efforts, and decide what's in the best interest for both of you. Also realize that whatever you do, someone will tell you you're wrong. Just hope it's not the GND controller you're working with.

ADDENDUM

An important note I did not include in the original publication of this page refers to ATC MANOPS 303.3, more specifically, the use of conditional clearances. A conditional clearance is one that includes a condition attached to a movement instruction. For example, "taxi across runway 11 after the landing DC9 passes". Such clearances leave a possibility for danger, especially in low visibility conditions. If only part of the instruction were received, it could lead the aircraft to believe he was cleared across the runway, and the paperwork associated with a runway incursion could be the least of your worries. This applies to the tower position, too, when it comes to having aircraft enter the runway for take-off. Don't tell an aircraft, "taxi to position after the B737 lands." As ATC, you wait until the B737 passes the taxiway where the waiting departure is, and then issue taxi instructions to allow him to take position on the runway. In fog, when the TWR can't see the aircraft on final or the one waiting in the taxiway, ATC should ask the waiting departure to report when the landing aircraft passes by him. Upon receipt of that report, taxi instructions may be issued to allow the waiting aircraft enter the runway.

ADDENDUM #2

In a topic published after this one was done, it was noted that in ATC MANOPS, helicopters requesting "air taxi" are to be issued clearances to do so, which means use of the term "cleared". This is in contrast to what was said above regarding ATC MANOPS specifically stated that controllers are not to use the term "cleared" in conjunction with taxi instructions on an airport. While I haven't discovered the reason for this, it remains that helicopters air taxiing get "cleared", while ground vehicles, fixed wing aircraft and wheeled helicopters taxiing on the ground are "taxied".




My e-mail address is moxner@nbnet.nb.ca for your comments, criticisms, complaints and praise. Again, thanks for taking the time to read!