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Reading through FAR AIM. I came across the ADSB section which stated that transponders should be turned off except for the lead plane. Why is this?

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Because otherwise, being so close together, they would all have their transponders trigger at the same moment in the sweep, and all reply at the same moment, and all of the replies would arrive at the receiver at the same time, with the same strength, leading to a "garble" in which none of them could be reliably decoded and plotted.

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    $\begingroup$ Doesn't ATC also have automated systems for detecting dangerous situations? Those would lead to constant alarms for loss of separation. That might be another reason. $\endgroup$
    – Jörg W Mittag
    Jan 31 at 9:18
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    $\begingroup$ @JörgWMittag There is no situation where aircraft would deliberately fly that close together outside of an airshow or military use. ATC would have plenty of time to avoid the situation entirely, making the need to detect this situation pretty much zero. $\endgroup$
    – SnakeDoc
    Jan 31 at 15:57
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    $\begingroup$ This isn't really a thing. $\endgroup$
    – RetiredATC
    Jan 31 at 21:29
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    $\begingroup$ This is indeed the reason that this rule exists, but it stems from the time of Mode A/C radars. When the radar is a Mode-S radar and the aircraft are equipped with a Mode-S transponder it wouldn't matter. The radar would Selectively interrogate the aircraft one-by-one (roll call interrogations) and there would not be any garbling of the replies. The replies to All Call interrogation may be garbled if they are not suppressed by the Lock-out protocol. $\endgroup$
    – DeltaLima ♦
    Jan 31 at 22:51
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    $\begingroup$ Has anyone seen a formation flight breakup? Say you're working ACME21, a formation flight of 4 F15's that want to split into individual flights. You tell ACME21, "have ACME22 squawk 5252 and come up on my frequency." The radar has NO PROBLEM tracking ACME22, even though he's right next to ACME21 (and CA goes off immediately). @hobbs I don't know if you're getting this from a book or something, but in the real world, closely spaced transponders are not a problem. Have you ever seen a stack of aircraft holding for a major airport with overlapping targets? Same thing, no problem. $\endgroup$
    – RetiredATC
    Feb 1 at 1:54
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In the US enroute environment, a formation flight is controlled through the flight leader, which is assigned a discrete transponder code. Controllers separate a standard formation from other aircraft by 6 miles (instead of the normal 5) to account for the additional lateral dimensions of the formation.

The enroute computer system (ERAM) will detect conflicts between aircraft, including aircraft on a nondiscrete beacon code, and display an alert on the controller's radar display. The conflict alert (CA) is in the form of flashing full data block (on the tracked aircraft) and flashing limited data block (on the untracked aircraft). Since the formation is separating themselves from one another, having CA activated is unnecessary.

If a flight is a non-standard formation, it is customary to have the trailing aircraft squawk the nondiscrete subset of the flight leader (flight leader is assigned 5251, trailing aircraft will be told to squawk 5200). This enables the controller to add the appropriate lateral separation depending on the formation size.

Anytime a trailing aircraft has their transponder on and the CA activates, the controller can suppress CA to stop the flashing, but that only suppresses CA on their radar display. Other controllers can still see the alert if they have a full data block of the formation lead (which happens every time the flight transitions from sector to sector. Another option is the suppress group (SG) function. SG will inhibit CA in the entire facility (on aircraft added to the SG list) but will not suppress CA across facility boundaries. SG would be used for non-standard formation flights when the trailing aircraft is squawking a nondiscrete code.

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    $\begingroup$ EDIT: Added the phrase that SG only applies to the aircraft in the SG list. $\endgroup$
    – RetiredATC
    Jan 31 at 22:29
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To reduce the number of false air collision warnings

Straight from the FAA justification of the new rule, https://www.federalregister.gov/documents/2019/07/18/2019-15248/revision-to-automatic-dependent-surveillance-broadcast-ads-b-out-equipment-and-use-requirements:

During formation flight, the close proximity of aircraft and their data/identification tags displayed on the radar display can, at a minimum, clutter the ATC display making it hard for ATC to determine the exact location of the aircraft to provide appropriate separation from other aircraft. Additionally, an air traffic controller will receive repeated audio and visual alerts (flashing data tag) that aircraft are within close proximity to each other. These alerts can distract controllers and redirect their attention to aircraft with approved separation and away from other instances where the controller may need to provide control instruction to maintain necessary separation. In these cases, once aircraft are “joined up” as a flight, it is in the best interest of flight safety to direct subsequent “wingmen” in the flight to squawk stand-by or stop squawk since control instructions are provided to only the lead and there are established Start Printed Page 34285separation minima from formation flights. In the instance of non-standard formation, it is general practice to have the lead aircraft squawk, along with the trail/last aircraft, a subset beacon code with altitude. In order to minimize these conflicting or overlapping data reports, this rule allows ATC to direct only the lead aircraft flying in formation to transmit ADS-B or operate his or her transponder.

(Emphasis added to highlight relevant sections.)

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The whole flight is controlled as a single entity by ATC - hence all communications including transponders are only done to the lead plane.

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    $\begingroup$ do you have any source for the "the whole flight is seen as a single entity by ATC"? $\endgroup$
    – Federico
    Feb 1 at 9:30
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    $\begingroup$ @Federico The FAA's Air Traffic Control Handbook JO 7110.65z states in paragraph 2-1-13 "Control formation flights as a single aircraft." $\endgroup$
    – 757toga
    Feb 1 at 18:29
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    $\begingroup$ Controlled as, yes. Seen as, not necessarily. $\endgroup$
    – randomhead
    Feb 1 at 22:49
  • $\begingroup$ I can't speak for terminal radar systems (perhaps @757toga can) but in a standard formation, with the flight leader squawking a discrete code and the rest of the formation squawking standby, you will not see a primary target on the non-leader aircraft on an enroute display. As the primary radar feeds data into ERAM, the tracking software compares the primary radar tracking with the secondary radar tracking (or ADS-B data) and suppresses primary target display on aircraft that have a roughly matching secondary or ADS-B target. $\endgroup$
    – RetiredATC
    Feb 2 at 6:15
  • $\begingroup$ When I worked at the TRACON (many years ago) we could see the primary targets on the non-leader aircraft who were squawking standby. $\endgroup$
    – 757toga
    Feb 2 at 14:32

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