Review: "One In One Out Rule" at Class D Airports

[Zachary Beard]

This rule is the most restrictive and conservative method of IFR separation by a radar facility overhead a Class D facility.

That being said, let this serve as a reminder of how IFR operations work at Class D airports:

Class D control towers can separate IFR operations within their airspace by visual separation as long as standard IFR separation exists prior to and after the application of visual separation.

However, in real life Letters of Agreement are sometimes much more restrictive than this based on what type of facility overlies the airport and what radar equipment is in use by them. Because of this, the rest is a review of the most restrictive/conservative method of IFR separation in the Class D tower environment.

Class D towers must report to the radar facility overhead (i.e. DEN_APP when working APA_TWR) when an IFR arrival lands.

   - Ex. "Denver Approach, Centennial Tower." "Denver Approach." "N123 has landed." "ZY" "MM"

This tells the radar facility that they may clear the next IFR arrival for approach into the airport, or issue a departure release to an IFR departure who is waiting.

That is also why we must obtain an IFR release for all IFR departures. Ex. "Denver Approach, Centennial Tower." "Denver Approach." "Request release N123 to Grand Junction." (Include the runway number if more than one is in use at your airport as advised to the radar facility and within the ATIS). "N123 is released." "JW" "DM" (The release may include a heading to fly, or an amended initial altitude. If not, it is the tower's discretion to assign a heading if a pilot nav SID is not being flown).

If the radar facility has an IFR arrival inbound that is established on an Instrument Approach Procedure (IAP) they may issue "N123, hold for release" or "N123, expect departure clearance 0130 Zulu". It is the radar facility's job to call the tower back to issue the release when able; generally when the tower calls to advise the previous IFR arrival has landed.

Tower provided visual separation:

   - If the radar facility sees fit, based on the weather and traffic situation, they can delegate responsibility for utilizing visual separation between two specific IFR operations to the tower. Ex. "Visual separation approved between N123 and N456, N456 released at your discretion."

Voila! Advise if you have questions.

ZY

[Erik Partridge]

Having trouble finding this in the 7110.65 (no HTML version is freaking annoying). Would someone mind giving me the section for my own edification?

[Merik Nanish]

To my knowledge, the "one in one out" rule is only mandatory for IFR operations at UNCONTROLLED fields. It means if Approach clears an IFR aircraft to land at an airport, it cannot allow another IFR aircraft to depart at the same time. It must wait for the landing traffic to report on the ground (or missed approach) before releasing the other traffic for departure. I have never read anywhere in 7110 that Class D operations must also follow this. The fact that many of them do could be a letter-of-agreement kind of thing.

Visual separation cannot be applied for final approach at uncontrolled fields. Para 7-2-1 a-2 indicates that pilot applied visual separation can only be used if communication is maintained with at least one aircraft; you cannot do that with uncontrolled field operations, because the aircraft MUST be transferred to advisories for final approach. This is because the aircraft must announce itself on CTAF frequency, so other IFR/VFR traffic in the vicinity of that airport can maintain separation.

I am eager to see if there is another FAA documents that specifies this is mandatory at all Class D airports.

[Zachary Beard]

This is assuming the tower doesn't have a certified radar display, which in my experience, most Class D towers don't have.

Merik: the visual separation mentioned is tower-provided visual separation, and must be approved explicitly between two aircraft by the radar controller, and delegated to the tower.

[Chris Robison]

This is assuming the tower doesn't have a certified radar display, which in my experience, most Class D towers don't have.

Can you provide references to this information?(AIM, 7110.65)

[Zachary Beard]

Something at work confused me. Edited the post to clarify.

Here at the academy in the Enroute domain students practice issuing IFR releases to a Class D nonradar tower. Unsure of the letter of agreement between the center and the tower, but students are taught to effectively use the one in one out rule for IFR operations at this airport.

Likely it's just a letter of agreement thing. Class D towers in general can provide IFR separation utilizing visual separation if standard IFR separation exists prior to and after visual separation is used.

Discussion point for staff members: How restrictive or detailed are your LOAs between your Class D fields and their overlying radar facility? At ZDV I am considering developing a standard procedure for IFR operations at Class D fields to make it consistent.

[Bradley Grafelman]

This is assuming the tower doesn't have a certified radar display, which in my experience, most Class D towers don't have.

In addition, the cab controllers would have to be likewise certified/trained to use the CTRD for the purposes of providing a radar service (namely radar separation). Not sure if the two are always guaranteed, but I could see how they wouldn't be (and a cursory glance at some admittedly dated StuckMic posts would lead one to believe this as well).

Can you provide references to this information?(AIM, 7110.65)

One reference might be 4-8-1(g) for subsequent approaches. A broader reference would be the separation minima for IFR-IFR in Class D airspace. If the radar controller has to maintain separation, and he can't transfer that responsibility over to tower in the form of radar separation, how can he guarantee the separation without applying one-in-one-out or other nonradar techniques?

ZY: I was half expecting you to suggest everyone brush up on their timed approach knowledge as well.  

[Merik Nanish]

So when you say "certified radar display", which type of radar is considered certified? DBRITE, RACD, LRAC?

Also what you say in terms tower-applied visual separation makes sense. Thinking about it, if whether is IMC, at all airports (regardless of being Class B, C or D ) you must apply the one-in one-out except if the tower has approach radar (LRAC), right? And because most Class B (all of them?) and many class C airports have LRAC, you only mentioned Class D to begin with?

My point is: I don't think the one-in one-out rule has anything to do with what airspace the airport is in (Class B, C, D, E, G). It has to do with the Tower's ability of providing separation. And the tower/pilot's ability to provide/maintain separation depends on VMC/IMC as well.

[Bradley Grafelman]

So when you say "certified radar display", which type of radar is considered certified? DBRITE, RACD, LRAC?

FWIW, KAPA (Class D) has a RACD, and the D01-KAPA LOA allows for tower separation criteria that include radar distances "utilizing the RACD."

[Ryan Geckler]

My point is: I don't think the one-in one-out rule has anything to do with what airspace the airport is in (Class B, C, D, E, G). It has to do with the Tower's ability of providing separation.

We have a winner!

[Zachary Beard]

Yay learning!

[Wesley Miles]

Zach,

We're implementing something similar at ZKC, but it's strictly for Class D's directly under Center airspace, where RADAR coverage isn't to the ground, and simulating those towers being "VFR Towers" without a DBRITE, R-ACD, etc.  

One-in-one-out is a method of non-radar separation.  It is used at uncontrolled airports, and Class D's where- as you said- the tower doesn't have a radar display, and/or the overlying facility doesn't have radar to the ground.

[Zachary Beard]

Good idea Wes, thanks for the heads up. We might do something similar at ZDV.

[Jason Baxter]

Rather than using the one-in-one-out rule for uncontrolled airports, I've been using timed approaches. So long as you have WX above the circling mins you should be good to go. Perhaps we should be using this instead since it requires no special circumstance or training on the part of the tower. Or is there some reason that this method is not preferred?

[Dhruv Kalra]

Don't forget that under VMC or as long as the tower can provide visual between an arrival and a departure, you can provide a "released subject your discretion" release along with any relevant control instructions as well.