What Is In-Flight Material (Free Fall) and How to Set It?

The scale has reached its target and the gate has closed — but the weight keeps rising. After the gate closes, material still hanging in the air between the bin and the scale falls onto the scale by gravity. On site this is called “in-flight” material; in the literature it is known as “free fall”.

If this amount is not taken into account, every batch ends up above target. It looks small, but added up over a shift it becomes a serious number.

Why is it different for every bin?

The in-flight amount is not a single number; it is different for each feeding point. It depends on:

• Drop height — the longer the distance between gate and scale, the more material is in the air
• Gate opening and flow rate — a wide-open gate lets more material through per second
• Material type — gravel flows freely; fine sand and filler come down more slowly and cling
• Moisture — wet sand forms lumps and its flow changes
• Gate closing time — the slower the pneumatics, the more material in flight

That is why the value for 0-5 sand and 12-22 gravel is never the same. It is measured separately for each bin.

The fix: close the gate early

The logic is simple. The gate is not closed when the target is reached, but earlier by the in-flight amount:

Gate close point = Target − In-flight

If the in-flight amount is measured as 18 kg and the target is 645 kg, the gate closes at 627 kg. When the 18 kg in the air lands, the scale settles at 645.

How is the value measured?

During commissioning, separately for each bin:

1. Set the in-flight value to zero
2. Tare the scale and weigh that bin alone to its normal target
3. Read the final value after the weight has fully settled
4. Overshoot = reading − target. This is that bin’s in-flight amount
5. Repeat at least 3 times and take the average
6. Enter the value in the settings and confirm with one more weighing

Do not settle for a single measurement; material flow changes from batch to batch, so averaging is essential.

What is jog feeding for?

In-flight compensation alone may not be enough — especially for small, sensitive quantities such as cement and admixture. Here a second mechanism comes in: near the target the gate opens and closes in short pulses instead of flowing fully open.

The last kilos arrive slowly and under control, and the risk of overshoot drops. It is usually not needed for aggregate; it is recommended for cement and admixture.

When should it be readjusted?

• When a gate or pneumatic cylinder is replaced
• When air pressure changes permanently
• When the material supplier or fraction changes
• At the change of seasons (moisture changes the flow)
• When reports show a systematic deviation on a particular bin

The last point matters: logging deviations per bin is the only thing that tells you when the setting has drifted. If you cannot tell out-of-tolerance batches apart in the report, you only notice the problem after the concrete has been poured.

And the opposite: when material does not flow

In-flight material is the problem of going over target. There is also the problem of staying under: if the gate is open but the weight does not increase, the silo is empty, the material is bridging or the gate has not actually opened.

The software must detect this by watching the weight increase over a set time and stop production. Otherwise the batch is completed with missing material and nobody notices.

Automation where you can set in-flight compensation bin by bin, with jog feeding and flow monitoring: DKT CPlants. Related article: how to calculate moisture correction in a concrete batching plant.