So far, as regards the engine installation, the performance has been focused on the fuel consumption. During its operational life, a gas-turbine engine might be required to operate outside of normal continuous limits. In such circumstances, it will have limitations placed on it which are determined by the permissible operating temperature of the turbine section. So if the engine is required to operate at a power above the normal continuous limit, providing this occurs for a specified limited time period, it is possible to achieve this without causing
permanent damage. It may happen that a situation arises which can be considered to be an emergency and in order to save the helicopter, excessive wear or indeed damage to the engine(s) may be the only possible choice. These are also catered for, but the time limits are necessarily short. To illustrate this, typical examples of such power limitations are as follows.
A.5.1 Maximum Continuous Power Rating
This is the maximum power at which an engine can operate continuously. Consequently, it does not have a time constraint.
A.5.2 Take-Off or 1 Hour Power Rating
This rating is applicable for the higher power situations such as operation at high altitude and/or ambient temperature and particularly for take-off and hover. Time limits of approximately 1 hour (sometimes V2 hour) are allowed before the engine must revert to a lower power setting. (A working figure is 10% above the maximum continuous rating.)
A.5.3 Maximum Contingency or 21/2 Minute Power Rating
By its title, this power rating is used in contingency situations, such as the loss of an engine. The time limit is considerably shorter and usually for a period of 2 to 3 minutes. (A working figure is 20% above the maximum continuous rating.) Because of the high level of power increase it is quite possible that an engine inspection be considered.
A.5.4 Emergency or 1/2 Minute Power Rating
This is a rating used only as a last resort, when saving the helicopter is the priority. Engine damage is a real possibility for this situation. The time limit is very short (30 seconds) since engine failure is a real consideration. (A working figure is 30% above the maximum continuous rating.)
To illustrate the need for such an excess of power the following situation is provided as an example. Consider a twin-engine naval helicopter which suffers an engine loss in a condition requiring high power – hovering at a high all-up weight, for instance. If this occurs over the sea then the pilot may be forced to lower the helicopter onto the sea surface. To achieve a take-off from the sea with an engine lost will require a reduction in all-up weight, so jettisoning as much weight as possible will be necessary. Emergency power will be required for a take-off on a single engine from the water. After retrieving the helicopter from such a dire situation and returning to base/ship, the engine(s) will probably require extensive maintenance and refurbishment. The damage may be such that it is beyond repair and will need to be scrapped.