What are the constraints and procedures regarding IMC formation flight?

Question

"Instrument Meteorological Conditions" are those in which flight under Visual Flight Rules is not permitted. This changes based on airspace but for the most part IMC means that flight visibility is lower than either 3 SM or 5 SM.

A "standard formation flight" is one in which all elements maintain no more than 1 NM laterally and no more than 100' vertically from the flight lead. Clearly a standard formation could be carried out under IFR if flight visibility were lower than 3 SM. But is there a lower limit adhered to by any organization? Demonstration teams are often said to fly with mere inches between their wingtips; visibility would have to be extremely limited to make that impossible, albeit much more risky, or so I imagine.

Furthermore, do all cloud types and instances generally restrict visibility the same amount? Or is there variability between types, or even intra-type on different days?

Answer 1

Short answer: The constraints are that you maintain sight, and procedures are to train and practice regularly, and thoroughly brief all contingencies.

CONSTRAINTS Visibility can vary significantly in different cloud types, and even within a layer. Cumulus are generally the most dense and therefore have the poorest visibility. They are generally the most turbulent as well, which makes compensating by flying closer more of a challenge.

That said, there is no real lower limit or minimum visibility for scheduling operations as it would be impossible for a ground station to measure or forecast.

The reality is that the lower limit is established by the comfort level of the wing pilot. You tuck it in as close as you are able to when the clouds get thick, and for several minutes your entire world may shrink down to the immediate and life-affirming task of holding a small airfoil shaped wingtip or nav light in a constant relative position on your canopy. You might be doing this while getting bounced around and having lost your other positional reference points because the clouds are so thick you can no longer even see the fuselage of the lead aircraft.

This is where a having an NFO, (Naval Flight Officer) with a soothing voice in the right seat is worth its weight in gold. While you fight to keep from either losing sight or hitting the other plane, your situational awareness is enhanced by little sugar calls on the intercom like "in an easy right turn to base leg now, we should be rolling out in about 10 more degrees... passing 4000 for 3000 at about 800 FPM down..."

Sometimes that wingtip will even flash in and out of sight briefly as you transit the thickest sections, anything more than about a second or so should trigger execution of lost sight procedures, which takes me to...

PROCEDURES Lost sight under IFR is an emergency because you suddenly have an aircraft, (the wingman) operating in IMC, without an IFR clearance, WELL inside of ATC separation requirements. So, you can’t count on ATC for the immediate separation needs, you need to take action.

Lost Sight procedures are briefed on every formation flight, and they need to be deeply ingrained and automatic so when that small piece of metal 10-15 feet away, (that's attached to a much larger piece of metal 30-40 feet way) suddenly disappears from view there isn't any confusion about what to do.

The important thing is for the lead to remain predictable and smooth, continue to comply with the IFR clearance, and for the wingman to take action.

Specifically:

• Climb or descent - Lead continues to assigned altitude, wingman adds power (if in a descent) or pulls power (if in a climb) to level off.
• Turn away - Lead continues the turn, wingman levels off to achieve a heading split.
• Turn into - Especially important for the wingman to call "lost sight" on the radio, and over bank to get away. Lead should level wings until a 30 degree heading split is established, then resume turning to assigned heading if being vectored.

There are additional techniques that may be covered in more detail depending on the circumstances, and/or experience levels of the crews or number of times they have flown together.

For example, if I was the formation lead pilot and was briefing with a new pilot/NFO crew I hadn't flown with before I might add some mission specific details like "there's a pretty thick layer on the way to the MOA, if you lose sight in the climb call it out on the inflight freq and I will keep climbing, calling every thousand feet as I pass. Get on the radio with ATC and pick up your own squawk, and ask to resume climbing once we have at least a thousand foot split. I will let you know as soon as I find VMC, and audible a rendezvous plan on the inflight freq."

Those who fly together regularly start to think alike, and you would likely spend 3-5 seconds reading the bulleted points above straight off the mandatory items on the briefing card, and then move on with the rest of the brief.

Answer 2

Two very different types of IMC formation flying that I had some experience in, with fairly different criteria. Both of them in the military.

First is the very close formation -- what we called "fingertip" formation in the training command, which we flew in T-37's and T-38's, and basically what you see when the demonstration teams are tucked in tight. While the specifics would differ from airplane to airplane, the basic idea is that the #2 wingman is slightly aft of lead, and his cockpit is probably within one wingspan of lead's fuselage (i.e. the wingtips overlap in left-right aspect, but are separated fore-aft and up-down). When you consider how thick clouds would have to be to not see from one side of a fighter or trainer aircraft to the other, especially when lead has some lights on in the most useful configuration, that's not common.

The other place that this sort of formation can be useful is in air refueling... the receiver is behind and below the tanker, but within the length of the boom (once hooked up), and there are lights on the tanker's belly. While there are procedures for lost-sight in all of these cases (lead goes faster & up, receiver or #2 goes slower & down, briefed turns in many cases, more complex for a larger formation), with experience it becomes reasonably common for a 2-ship or a 4-ship to fly around in IMC, for all purposes as a single aircraft. The "standard formation" definition has meaning for ATC, but for the formation itself you're in fingertip, or something with slightly more space (probably not used in IMC), or some tactical formation (much more space, wingman can pay attention to things beyond just holding an exact position on lead), and whether or not it is "standard formation" is an afterthought.

The other type of formation that works in IMC is what in the C-130 world we call SKE, for Station Keeping Equipment. The basic technology is really old, and is more-or-less TCAS version 0.1 between cooperating aircraft only. It used dedicated equipment, not transponders, and had a whole host of parameters & procedures & some limitations (especially with large formations), but allowed us to fly at 4,000' trail and co-altitude. That very quickly (at least once you got beyond one wingman) became non-standard formation for all ATC purposes, and was a long string of aircraft in the sky. You'd follow lead, he gets a vector, and when the wingman got to the same point in space, he makes the same turn. We could fly a formation ILS that way; once you intercepted the localizer, each aircraft flew the ILS course & vertical path, but kept spacing on the aircraft ahead. It was ponderous, but it worked.

The current version of SKE, from all I've heard, is a world ahead of what we had in the 1990's and early 2000's: far better datalink, integrates with the autopilot much better, etc. Pretty cool what you can do with 2020-era digital technology compared to 1970's era stuff!

Other aircraft could do something similar using air-to-air TACAN or air-to-air radar. I'll let somebody more familiar with those procedures add details.

But in both the case of fingertip formation and SKE formation, you could fly an ILS down to minimums, and land the whole formation on the same runway in one pass... the "wing landing" might get interesting in really low vis, but single-seat jets don't do Cat II or III, so nothing crazy low by the time you're on the runway. The SKE formation landing ended up with (IIRC) about a mile between aircraft, so you could have more than 1 on the runway at once, but there were procedures to make that work, and it generally did work quite well. Equipment getting temperamental was the most common issue.

Good times, but not really anything I feel the need to get back into with the 737!