Appearance
Time-based separation
Most VATSIM controllers have only ever separated arrivals by distance: a Heavy behind a Super gets 6 NM, a Medium behind a Heavy gets 5, and so on. Heathrow has worked differently since 2015. Time-based separation (TBS) keeps the time between arrivals the same whatever the wind does, and converts that time into a distance for you. This page explains the idea and how Intelligent Approach works it out.
Why distance alone falls short
Wake turbulence separation exists so the follower meets the leader's wake only after it has had time to decay and drift down. What matters to the wake is the time between the two aircraft, not the distance.
A headwind slows every aircraft over the ground. With distance-based separation (DBS) the distance stays the same, so in a headwind the follower now takes longer to cover it. Each pair ends up further apart in time than the wake needs, and fewer aircraft land per hour, exactly on the windy days when the queue is already long.
TBS removes that loss. It takes the time the wake minimum gives in light winds, and keeps that time in a headwind. Because the follower is slower over the ground, the same time is a shorter distance.
In one sentence
TBS asks: "How long does this wake minimum take to fly on a calm day?" and then "What distance takes that long today?"
How a time becomes a distance
Every TBS figure on your panel comes from three steps.
1. The reference time
Start from the distance-based wake minimum for the pair (RECAT-EU, or pair-wise where that applies). Work out how long the follower takes to fly that distance to the threshold in still air. That time is the TBS minimum for the pair.
To do that you need a speed profile for the final approach. Intelligent Approach uses the profile from the EUROCONTROL guidelines:
| Segment | Speed |
|---|---|
| Beyond 4 NM | 160 kt on the glide |
| 4 NM to 2 NM | Slowing steadily from 160 kt to the final approach speed |
| Inside 2 NM | Final approach speed |
2. Today's headwind
The plugin builds a headwind profile down the 3° glide path: the headwind component every 0.25 NM out to 10 NM, at the height an aircraft would be there. Ground speed at each point is the profile speed minus the headwind.
3. Back to a distance
The plugin then finds the distance which, flown at today's ground speeds, takes exactly the reference time. In a headwind that distance is shorter than the DBS minimum. A small buffer is added on top, because the wind changes and aircraft don't fly the profile perfectly.
A worked example
A pair with a 5 NM wake minimum, on a day with a steady 20 kt headwind down the approach:
- Still air: the follower takes about 125 seconds to fly the last 5 NM.
- Today: flying 20 kt slower over the ground, 125 seconds covers only about 4.2 NM.
- Buffer: at 20 kt of headwind the plugin adds 0.5 NM, so the separation to deliver is 4.7 NM.
The same pair would need 5 NM under DBS. Over a busy hour that recovered 0.3 NM per pair adds up.
What the wind does to the numbers
What Intelligent Approach delivers for some common wake minima, with a steady headwind down the approach (rounded to 0.1 NM):
| Wake minimum (DBS) | 10 kt headwind | 20 kt headwind | 30 kt headwind |
|---|---|---|---|
| 3 NM | 3.0 | 3.0 | 2.9 |
| 4 NM | 3.9 | 3.9 | 3.6 |
| 5 NM | 4.9 | 4.7 | 4.4 |
| 6 NM | 5.8 | 5.6 | 5.2 |
| 7 NM | 6.7 | 6.5 | 6.0 |
| 8 NM | 7.7 | 7.3 | 6.9 |
Two things stand out. The bigger the separation, the more TBS recovers. And the smallest separations barely move: the buffer eats most of the gain, and the radar separation minimum underneath still applies (see below).
The rules around it
Light winds and tailwinds: back to DBS. TBS only applies once the average headwind down the approach reaches 5 kt. Below that, or in a tailwind, the plain DBS distances apply. The panel tells you which mode is in force.
Never more than DBS. In a tailwind the time equivalent would be longer than the DBS distance. The delivered separation is capped at DBS, so TBS can only ever reduce a wake distance, never add to it. The panel's TBS table still shows the uncapped figures for information.
Radar separation still applies underneath. TBS only changes the wake minimum. The radar separation minimum (RWY SEP: 2.5 NM at Heathrow, 3 NM at Gatwick) is a floor that TBS never goes below.
Runway occupancy is spacing, not separation. The time the leader needs to vacate the runway (ROT) is converted to a distance with today's wind too, and the larger of the two governs. On the panel, pink means separation (wake or radar) governs and green means spacing (ROT, departures or an agreed gap) does. See Separation and spacing.
No live wind, no TBS. If the wind data is missing or out of date, the plugin falls back to DBS.
Delivering it: target distances and compression
TBS gives the separation needed at the threshold. But aircraft slow down on final, and the leader slows first, so a pair spaced exactly right at 8 NM will have closed up by the time the leader lands. This is compression.
The real system, and Intelligent Approach, give you markers that already allow for it:
| Marker | What it is |
|---|---|
| FTD (final target distance) | The separation required at the threshold. |
| ITD (intermediate target distance) | Where the follower should be now so that, with both flying the standard profile, it arrives at the threshold exactly on the FTD. It sits further back than the FTD while the leader is still fast, and closes in to the FTD as the leader lands. |
Fly the follower onto its ITD at 160 kt and the compression is taken care of. See Final approach markers for how they look on the scope.
Where Intelligent Approach differs from the guidelines
The EUROCONTROL guidelines describe how a real air navigation service builds TBS, calibrated on a year or more of local radar and wind data. A VATSIM plugin has none of that, so a few things are done differently.
| Topic | EUROCONTROL guidelines | Intelligent Approach |
|---|---|---|
| Reference times | Measured from real calm-wind approaches, per aircraft type or category | Calculated from one generic speed profile (160 kt, slowing between 4 and 2 NM to 135 kt) for every type |
| Wind | Measured from aircraft (Mode-S) reports over the last ~10 minutes, in 0.25 NM segments | A forecast weather model (surface and upper winds), refreshed every 10 minutes, sampled every 0.25 NM down the glide to 10 NM. Stale after 45 minutes, then DBS |
| When TBS applies | Locally defined, typically from about 5 kt of headwind, no tailwind | 5 kt mean headwind down the approach |
| Buffer | Set per wake pair and wind band from local data, in seconds, to a target error rate | A simple rule: 0.025 NM per knot of headwind, up to 0.6 NM |
| DBS cap | Wake distance including the buffer is capped at DBS | The conversion is capped at DBS, then the buffer is added, so a small separation can come out a touch above DBS (3 NM becomes 3.0 to 3.1 at moderate headwinds). This matches the figures on the real Heathrow panel |
| Delivery point | Set locally: the threshold, or up to 1 NM before it | The threshold |
| Compression model | Calibrated per type | A deliberately cautious standard profile: both aircraft at 160 kt, then slowing to a conservative final approach speed by 2 NM. A follower flown at 160 kt should not end up inside its target |
| Separation loss | Judged against the minimum | Judged against the TBS minimum without the buffer; the buffer is delivery margin |
Sweatbox and offline sessions
Live wind is only used once your connection is verified on the live VATSIM network, because sweatbox aircraft fly the simulator's weather, not the real world's. In a sweatbox you get DBS unless you force live wind with .ia wind on.