HEED stands for Helicopter Emergency Egress Device: a personal emergency breathing device for escaping from a submerged helicopter. It provides a small carried supply of breathing air intended to support immediate self-rescue from the cabin and the journey to the surface. The supply remains limited by the cylinder contents. Helicopter Underwater Escape Training (HUET) with an emergency breathing device links its specific handling to the underwater escape task.
Breathing air from a limited pressure cylinder
HEED 3 is a device family made by Submersible Systems. Its cylinders contain compressed breathing air. The regulator reduces the high cylinder pressure and delivers air through the mouthpiece on inhalation. The hoseless 175 and 300 Series use an integrated single-stage demand regulator. In the 200 Series hose model, a first stage reduces cylinder pressure to an intermediate pressure; a second stage at the mouthpiece supplies air at the surrounding pressure. These regulators also work sideways or upside down. An inverted cabin attitude therefore does not, by itself, interrupt air delivery.
In the configurations described, the 175 and 200 Series hold approximately 58 litres of air and the 300 Series approximately 85 litres, all at a service pressure of 207 bar. The litre figure describes the expanded air quantity rather than the internal volume of the small cylinder. Usable duration depends on the model, actual fill, breath volume and breathing rate, physical exertion, water depth and air losses. A calculated breath count assumes a fixed breath volume at the surface. Larger or more frequent breaths consume the supply faster. Unnecessary use of the purge button and leaks consume the same reserve from which the person breathes.
Access and readiness
The device must remain accessible in the equipment actually worn. Its holster needs enough clearance to grip the regulator and an unobstructed path for withdrawal. A stowed device whose grip or withdrawal path is blocked by other equipment cannot provide the person with air at the critical moment. With a hose model, the mouthpiece and hose must be able to pull free of their stowage.
Pre-use checks cover visible damage and broken or loose parts; the 200 Series also requires checking the hose for cuts and blisters. Damaged devices are removed from use. The hoseless 175 and 300 models have an always-on air supply and require no separate shut-off valve to be opened. The 200 model has its own on/off knob: its valve is fully opened for readiness before checking the pressure gauge and carrying out a brief purge-button test. Fill pressure is checked on the gauge; its needle should be in the green zone. A full cylinder behind a closed valve still cannot supply the hose regulator.
The mouthpiece and water in the regulator
With the hoseless HEED 3, the holster flap is opened and the device withdrawn by its regulator, releasing the mouthpiece cover. With the hose model, the regulator is pulled out until the hose is free. When a regulator is brought to the mouth underwater, water inside it must be cleared before the first inhalation. A brief press of the purge button or a sharp exhalation through the mouthpiece performs this task. Inhaling directly from a flooded regulator initially brings in water. The purge button releases air, so it is used only as much as clearing requires. Inhalation and exhalation then continue while the rehearsed escape proceeds.
If a HEED 3 is breathed completely empty underwater, water can enter the system. The device then needs an overhaul; a restored fill pressure alone does not restore readiness. Detected leaks and damage also need to be addressed before further use.
Device handling in underwater escape
In the US Marine Corps' Underwater Egress Trainer (UET) programme, the single-seat Shallow Water Egress Trainer (SWET) introduces submersion and the use of supplemental emergency breathing devices before, or alongside, exercises in larger cabin trainers. Device access, air activation and regulator clearing are thereby linked to the escape task. Extra breathing air can provide time for that task; it cannot open a blocked exit or provide orientation inside a dark, inverted cabin. Those problems remain part of the trained escape.
The terms belong to different devices and training systems. EBS means Emergency Breathing System and names the broader category of emergency breathing systems. The USMC programme names Survival Egress Air among its devices. At Yeovilton, the British Royal Navy uses Short Term Air Supply Systems, or STASS, for additional breathing time during underwater escape training. These names do not specify an identical cylinder capacity, valve or regulator arrangement. The wearer therefore needs to know which device is actually issued and how access, activation and air delivery work on that device.