In the UK (and I believe it's not uncommon elsewhere) landing clerance will never be given to an aircrat until it is number one and the runway is unoccupied.

 

In the states, this restriction doesn't seem to exist. Multiplie aircraft can be given clearance to land in sequence, or be cleared to land even with departing traffic on the runway. This doesn't seem to be a good idea, especially if comms are lost ( see this video:

) or the controller simply loses track of who they've cleared to do what.

 

Anyone know why it's done this way?

It's called "anticipated separation" and has been in use for decades. For arrivals, it means required separation will exist by the time the arrival crosses the landing threshold. For departures, required separation will exist when the departure begins takeoff roll.

 

Les Parson

I've seen this at KLGA. 3 consecutive departures off rwy 31. As soon as one was airborn, the next one would start its takeoff roll. By the time that next plane was airborn, the preceding plane would be a few miles ahead of it. Then, the next plane would start its roll. The aircraft were given different departure heading (eg 330, 310, 330) to provide additional separation once airborn.

It's called "anticipated separation" and has been in use for decades. For arrivals, it means required separation will exist by the time the arrival crosses the landing threshold. For departures, required separation will exist when the departure begins takeoff roll.

 

Les Parson

 

Yeah I understand that they're anticipating separation, what I don't get is why? What advantage does it provide over waiting until separation actually exists, given the reduction in safety?

One advantage is reduction in frequency congestion. For example, landing clearances are normally issued about at the FAF rather than short final during critical phases of flight. Of course this can be argued both ways indefinitely.

 

While the U.S. has adopted several European/ICAO ATC procedures and practices, anticipated clearance has remained.

 

As a former controller in Europe and major terminals in the U.S, (JFK, LAX), I prefer the U.S. method.

 

Les Parson

Yeah I understand that they're anticipating separation, what I don't get is why? What advantage does it provide over waiting until separation actually exists, given the reduction in safety?

Because it makes airport capacity acceptable to the airlines. Without it, there would be double the amount of aircraft in line on the taxiway waiting to go along with a reduced arrival rate. Like the previous poster also stated the frequency congestion is also reduced tremendously. I worked at KMSP for several years and without anticipated separation the place would have turned into a parking lot. The reason being that each runway was used for both arrivals and departures unlike some airports where a runway may be designated only for arrivals or departures. When runways were dry arrivals were spaced 4 miles apart at 170 kts crossing the FAF which gave us time to get a departure launched (sometimes two if we had just a wee bit more space) between each arrival. As soon as the landing aircraft crossed the threshold, a departure was put into position on that runway (LUAW these days) and told to be ready, traffic on a 4 mile final. Instructions to each landing aircraft were to turn left/right at the first available turnoff, no delay exiting, for departing traffic. Next arrival was then cleared to land and advised traffic in position would be departing prior to arrival. As soon as I saw the nosewheel of the arrival start to turn off of the runway I cleared the departure for takeoff. By the time the engines spun up and he was starting his takeoff roll the previous arrival had cleared the runway and the next arrival was about a 2 mile final. And over and over and over the sequence went.

 

Just like a fine tuned machine.....it was also likened to stuffing 5 pounds of crap into a 4 pound bag!

 

Best,

 

Bill

Because it makes airport capacity acceptable to the airlines. Without it, there would be double the amount of aircraft in line on the taxiway waiting to go along with a reduced arrival rate. Like the previous poster also stated the frequency congestion is also reduced tremendously. I worked at KMSP for several years and without anticipated separation the place would have turned into a parking lot. The reason being that each runway was used for both arrivals and departures unlike some airports where a runway may be designated only for arrivals or departures. When runways were dry arrivals were spaced 4 miles apart at 170 kts crossing the FAF which gave us time to get a departure launched (sometimes two if we had just a wee bit more space) between each arrival. As soon as the landing aircraft crossed the threshold, a departure was put into position on that runway (LUAW these days) and told to be ready, traffic on a 4 mile final. Instructions to each landing aircraft were to turn left/right at the first available turnoff, no delay exiting, for departing traffic. Next arrival was then cleared to land and advised traffic in position would be departing prior to arrival. As soon as I saw the nosewheel of the arrival start to turn off of the runway I cleared the departure for takeoff. By the time the engines spun up and he was starting his takeoff roll the previous arrival had cleared the runway and the next arrival was about a 2 mile final. And over and over and over the sequence went.

 

Just like a fine tuned machine.....it was also likened to stuffing 5 pounds of crap into a 4 pound bag!

 

Best,

 

Bill

 

I don't see how it makes that much difference to capacity as it doesn't have an affect on spacing. Just instruct arriving traffic to continue approach, then clear to land when the runway is clear. It won't increase traffic buildup on the ground as you can still use conditional clearances for departures (line up after the landing aircraft etc).

One reason is that a landing clearance, using your scenario, would not be issued until the arrival is over the approach lights or very near the runway threshold. Pilots get nervous and keep asking if they're cleared to land as they don't want to risk landing without a clearance. When we were able to get two departures out in between arrivals we also had to use minimum runway separation (SRS....6000 feet and airborne) so it left VERY little time to issue a landing clearance. It was all about expediting the operation and reducing freq clutter.

 

Bill

Just an aside, at the EAA fly-in at Oshkosh WI, arrivals are cleared to land three at a time where the clearance includes the color of one of three dots marked on the runway. Talk about busy!

What advantage does it provide over waiting until separation actually exists, given the reduction in safety?

 

What reduction in safety?

 

I can see how there's a perceived reduction in safety (a word I hate with the fieriest of passions*), but in the end, it evens out. In the current (US) scenario, there is a reasoning behind allowing high-flow procedures like this.

 

Anticipated separation: "AAL245, Runway 1 cleared to land, number two following a United 737, two mile final."

The pilots know they are cleared to land and can expect a go around if separation degrades. This is actually a lot easier on the brain. A is the expectation, but B may occur by prompt or by my initiation.

 

Worldwide standard: [potentially long wait] "AAL245, Runway 1 cleared to land."

The pilots likely assumed they were cleared to land, but did not know either way until closer in to the runway. This is harder on the brain, as no condition has been specified. A or B may occur by prompt, and neither is officially noted as expected.

 

Using anticipated separation, the pilot mental strain is lower because they know they have been cleared to land earlier rather than later. This also reduces frequency congestion, as the command can be issued with flexibility, rather than in a time-sensitive manner. As an example, if I know I need to issue a few departures and I have one inbound with room to get two out, I can clear the guy landing earlier and note traffic will depart prior to his arrival. From there, I can issue the time-sensitive (because of departure separation considerations) commands and not have to worry about issuing the landing clearance on a time-sensitive condition as well (in this case, that condition being the runway being clear).

 

Additionally, the amount of traffic that goes through the US airports is quite a lot more than you see elsewhere in the world, so this is a way that the system can be more efficient. Sure, there are world airports with more operations, but where else in the world do you have busy metroplexes? Elsewhere, if you back up one busy airport, the terminal area only gets messed up for that one airport. In the US, however, if you back up arrivals into LGA, you may also be affecting TEB, JFK, and EWR, which are also busy. So, if I'm running slightly higher separation because I can use anticipated separation and don't want to be issuing landing clearances when the aircraft is closer in, it ripples back. That means larger delays, and potentially more holding. Higher controller workload is the result of that (again, rippling back beyond the TRACON, and into the ARTCC). That higher workload can then lead to stress-induced mistakes, which would likely be more of a safety issue than simply telling the pilot: "hey, you're cleared to land, but there's one other guy in front of you."

 

Again, at that point, he knows he's cleared to land, but that the aircraft in front of him could possibly in the way of that happening. It really isn't at all less safe. It just seems that way because there's an overlap in who has been cleared to land on that runway.

 

Think of it this way:

There are (as of my last database pull) 19067 airfields in the United States without a control tower, while 709 fields have one. All of those non-towered fields are operating on anticipated separation for all intents and purposes. If you're thinking "okay, sure, but a lot of those airfields don't have a lot of traffic." That would be true. On the flip side, there are 5 non-towered fields (OAJ, SGU, ORS, LBF and MTJ) that see as much traffic as some of the top 85 towered fields. Those non-towered fields are clearly not having issues with anticipated separation, so is it really a safety issue?

 

Now, line up and wait (LUAW - thanks ICAO...my departure queue now sounds like a grocery queue) operations do lower safety margins slightly. This has led to a couple incidents, but most notably the USA/SKW incident at LAX. Safety is reduced because an obstacle is being placed on the runway, and is awaiting clearance to depart. If that is not received and the aircraft behind does not see the stationary traffic, but is cleared to land, a collision may occur. In anticipated separation (not using LUAW), an obstacle is being placed on the runway, but it is not being told to remain in that position indefinitely. In order to use anticipated separation with LUAW, the facility must have an approved safety logic system in place (similar to how RAAS works for aircraft).

 

For further reading, see "perceived safety" and "perceived risk."

 

 

*I hate the word safety because people rarely argue against the safety card, even when it makes no sense. Further, if you do argue, you're the dangerous one, and have an utter disregard for safety. Often, the safety card is pulled alongside the phrase "you can never be too safe," and all I can think of is yes, you can. Think Howard Hughes. The safest way to conduct life is to live in a hypoallergenic bubble, on a maximum security compound. Very few people take it that far, so no, "you can never be too safe" is clearly not the best motto, especially in aviation. The safest avenue would be to leave the plane on the ground.

 

Life is all about calculated risk. Knowing these methods of remaining safe, I feel confident that this risk (crashing, losing an engine, explosive decompression, flying into poor weather, etc), has been acceptably mitigated.

Multiple landing clearances are used in Canada at the busier airports as well and it work's very well. IMO it increases the margin of safety.

 

Off topic, the LUAW issue is different as most of the US incidents occurred during its implementation while it was still very new. I found many of the US pilots didn't know this phraseology change was coming and as such didn't understand what the term meant when issued it.

To us Europeans, it just doesn't make sense to give clearence to land, when they are not 'clear'

Anticipated separation: "AAL245, Runway 1 cleared to land, number two following a United 737, two mile final."

The pilots know they are cleared to land and can expect a go around if separation degrades. This is actually a lot easier on the brain. A is the expectation, but B may occur by prompt or by my initiation.

 

Worldwide standard: [potentially long wait] "AAL245, Runway 1 cleared to land."

The pilots likely assumed they were cleared to land, but did not know either way until closer in to the runway. This is harder on the brain, as no condition has been specified. A or B may occur by prompt, and neither is officially noted as expected.

 

I can see why you like anticipated separation, but the above is simply misleading. There is nothing wrong with saying "AAL245, continue approach, you're number two following a United 737, two mile final." That's no harder for the pilots following.

 

Additionally, the amount of traffic that goes through the US airports is quite a lot more than you see elsewhere in the world, so this is a way that the system can be more efficient.

 

Well I've checked, and Gatwick actually has the busiest single runway in the world, and they manage without anticipated separation, so I'm still not convinced. But then, this is just an informative dicussion and you've no obligation to of course!

 

Think of it this way:

There are (as of my last database pull) 19067 airfields in the United States without a control tower, while 709 fields have one. All of those non-towered fields are operating on anticipated separation for all intents and purposes. If you're thinking "okay, sure, but a lot of those airfields don't have a lot of traffic." That would be true. On the flip side, there are 5 non-towered fields (OAJ, SGU, ORS, LBF and MTJ) that see as much traffic as some of the top 85 towered fields. Those non-towered fields are clearly not having issues with anticipated separation, so is it really a safety issue?

 

In the UK, appart from the odd private airfield, there is almost always someone on the ground to talk to. However, not all of these are not 'towered' (staffed by an ATCO); some provide FIS (flight information service) or A/G (air ground radio), an even lower level of service. A/G is little more than an AWOS provided by an actual person. And I can tell you, I've definately flown into A/G staffed 'non-towered' airfields, and they are most definately dangerous! Not necessarily in terms of runway occupation, but in the traffic pattern was a huge free for all with lots of aircraft in it without any organisation what so ever. We may have to agree to disagree with this one!

 

Now, line up and wait (LUAW - thanks ICAO...my departure queue now sounds like a grocery queue) operations do lower safety margins slightly. This has led to a couple incidents, but most notably the USA/SKW incident at LAX. Safety is reduced because an obstacle is being placed on the runway, and is awaiting clearance to depart. If that is not received and the aircraft behind does not see the stationary traffic, but is cleared to land, a collision may occur. In anticipated separation (not using LUAW), an obstacle is being placed on the runway, but it is not being told to remain in that position indefinitely. In order to use anticipated separation with LUAW, the facility must have an approved safety logic system in place (similar to how RAAS works for aircraft).

 

Isn't LUAW required to get the most runway utilisation though?!

 

*I hate the word safety because people rarely argue against the safety card, even when it makes no sense. Further, if you do argue, you're the dangerous one, and have an utter disregard for safety. Often, the safety card is pulled alongside the phrase "you can never be too safe," and all I can think of is yes, you can. Think Howard Hughes. The safest way to conduct life is to live in a hypoallergenic bubble, on a maximum security compound. Very few people take it that far, so no, "you can never be too safe" is clearly not the best motto, especially in aviation. The safest avenue would be to leave the plane on the ground.

 

Life is all about calculated risk. Knowing these methods of remaining safe, I feel confident that this risk (crashing, losing an engine, explosive decompression, flying into poor weather, etc), has been acceptably mitigated.

 

Definately agree with you on this one. "If you spend £X million to save one life, it'll be worth it!". Really? Then I'll send you the bill!

What is a "stuck mic?"

What is a "stuck mic?"

 

It's when the transmit button sticks on your comm radio - EXTREMELY frustrating to any and all in comms distance on your freq... Some if the stuff you hear during these times is UNBELIEVABLE...

 

OBTW - I hate "Line up and wait"...

 

Regards,

Scott