Unreliable flow often occurs when an arch breaks or the rat-hole corrupts. The powders then pick up air and become aerated, leading to a flow which is usually in an uncontrolled manner (flooding, see Figure). Flooding sometimes gives completely uncontrolled and very fast discharge that in some occasions has caused drowning of people who did not manage to run away before they were covered by powder.

Often an effective way to overcome these phenomena is through the introduction of an amount of controlled air using an air permeation system. The location as well as the quantity of air must be closely examined; otherwise too much or too little of air will cause either a flooding condition or will not be sufficient to achieve the desired flow discharge.

Fig. Flooding and aeration