Missed approach protection

[stonethrower]

We seem to be considerably hampered in NZ by a requirement to protect the missed approach for arrivals versus preceding departures. In many cases it involves an aircraft having to roll before an arriving aircraft hits around 5NM final. This hammers our runway utilization as missed approaches normally continue straight ahead which is the same track as a preceding departure SID. Can anyone let me know how you gain efficiencies around this phenomena worldwide for single runway operations. It is embarrassing to tell an aircraft they'll be departing behind a slow aircraft on 6NM final when there is plenty of room to get one or even two aircraft out ahead. Our regulatory body and subsequently our Manual of ATS doesn't let us amend a published missed approach procedure with a heading to avoid conflict. :ugh:
Thoughts/examples/remedies all appreciated….

[fujii]

I was covering missed approaches in the sim today with two trainees. The Australian MATS makes provision for uncoordinated vectors during a missed approach to maintain separation with preceding traffic. Had to do it a number of times operationally and now covering it with trainees.

[zoneman]

Yeah, reduced separation in the vicinity of the aerodrome is the most adequate solution in most cases. Especially when it comes to putting faster a/c to roll in front of arriving slower a/c.
It might sound unsafe and ridiculous, but at the aerodrome where I work it often depends on controller's guts whether a/c will take off or wait 3-4 minutes more due to arriving trfc. That's the challenge of being ATCO, isn't it?

Is there a possibility to change Mapp at your a/d as to make a turn at appropriate distance/altitude, in order not to "follow" departing a/c?

[Tarq57]

Sorry in advance for the long post.
The issue is complex.

Well, not really, but there are several combinations of factors at play, which produces more complexity than needs be.

I've been working at the (probably) most-affected aerodrome for a few years, Wellington. At this field, the difference between the good weather traffic rate, and the bad weather traffic rate, is disproportionally high.


When the weather is suitable, which is a reasonable percentage of the time, reduced separation using visual separation in the vicinity of the field is the default position, and always has been. In these conditions, it's just another aerodrome, using runway separations, and if someone on final overshoots, they are directed into the circuit, and watched closely in reference to any prededing departure to ensure there is no risk of collision. If there is a big performance difference, a "bit more" than normal runway separation is generally used to ensure the catch-up will not result in excessive closure prior to the overshooting a/c turning into the circuit.

Standard, aerodrome-control-101 stuff.

When any element of the visibility or cloud base precludes directing an a/c into the circuit, standard distances are built in to the departure spacing required. For a jet departing ahead of anything, this minimum distance is 4nm. For a prop (DH8, B190, AT75 etc) is departing ahead of a similar, the distance is between 4 and 6 miles, depending on the runway, and the SID. When a lower performance prop is departing ahead of something on final, the distance can be anywhere from 4 to 12 miles, depending on the traffic mix, the SID, and the runway. 12 is quite a lot. It's required several times a day. So guess what? The slower a/c wanting to depart often gets bumped to a position in the queue that will minimize delays to the others.

Minimum vectoring altitude is 3-4000 on RWY34; 2-4000 on RWY16, depending on the direction of turn. In a situation where the traffic combo is 'adverse', (jet overshooting behind a light twin, or a Caravan etc departure) a lot of catch-up can occur before a turn can be made. Without the required gaps being applied, this would result in a loss of radar separation in most instances. Possibly a serious loss.

In certain weather conditions (stormy, basically, or cloud base/vis marginal) an overshoot is fairly likely. At a guess, I'd say such conditions happen at Wellington maybe fifty days of the year. (Pure guess on my part, but it's a ball-park figure.) So it's definitely a factor.

Within the past year or two, the circuit minima have been changed by CAA. The minimums for a controller to direct an aircraft into the circuit have increased, currently for jets it's 2000' base, 8K vis., day only. For props, it's 1500' base, and (I think) also 8K vis. (I should know. They keep changing it, though, so it's hard to keep the current figure mentally nailed without a document to hand.)
For both groups, there must be no severe turbulence, windshear, or "strong" crosswind reported.
Raising these minimums has meant that the percentage of days when visual separation can not be applied to protect the overshoot has increased significantly.


Interestingly, the pilots seem to use a different set of figures to determine whether a circuit is acceptable - the circling mimima displayed on the landing charts, which are considerably lower than the figures quoted above. Obviously there's a big difference between a circling manoeuver, and a circuit. Practically, however, they amount to the same thing.

In either case, a pilot is required to notify ATC before commencing an approach if a circuit is unacceptable. When this happens, the appropriate speed/gap is used in respect of a departure ahead.

There was recently an event when, on a "good weather'' day, an aircraft received an onboard windshear alert. (Not too surprising, as there had recently been a wind/runway change. We're not talking about big wind values, either, but enough to produce an alert, I guess.) The alert occurred at about 5nm final. The company involved have it in their ops manual that such a windshear warning requires a compulsory overshoot into the standard missed approach-no circuiting. An incident resulted in respect of preceding departing traffic.
Following discussions between management and the company concerned, the "patch" is consider all their operations to require the missed approach, so any time one of their aircraft approaches, it is subject to the "bad weather" standard speed restriction, and the departures ahead of it are restricted accordingly. This airline operates through Wellington several times a day.


The OP appears to consider that the CAA requirement to protect the missed approach to be archaic and unnecessary. Perhaps it is. I don't know. I tend to think it's not in the interests of safety, nor in interests of the ATC profession to be regularly setting up a situation where a loss of separation is fairly likely if someone overshoots for any reason. It doesn't happen that often when the weather is reasonable, maybe only once a month, but if it happens, and it's a real slow a/c departing ahead, and the "perfect storm" of factors is at play, it suggests a possible situation where TCAS is the only thing available to prevent a collision.

Would the industry find that acceptable? I suspect some within the industry would be ok with it. It would definitely help with on-time performance etc. I also suspect that most of the people in the pointy ends of aeroplanes might be a little less comfortable with it.


I've seen the "wheel" of this situation turn a complete rev a number of times over the past 30 years. I have to say that efficiencies have been achieved in some respects regarding the size of the required departure gaps. I also consider that there is quite a lot of wasted airspace as a result of the procedures we have. It is a bit frustrating to see quite a lot of sky "wasted", while aircraft wait at a holding point, as a result of a strategically designed solution to a problem which (I think) could benefit considerably from a more tactical approach at the point.

However, aside from possible "tweaks" in our system (which would probably increase the traffic rate by only a little, and then only in certain conditions), any change is only going to come about (1) if the industry demands it with sufficient stridency, (2) if CAA relax the pertinent rule.

I think the latter is quite unlikely.
The former is only likely if demand picks up, which will then result in exponentially-increasing delays.

As to what will be done? Anyone's guess. I await the re-invention of the wheel. Again.