Throttles above or below 80% HP - correct procedure?
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Reading through the manuals, I'm a little lost as to the procedures on maintaining a minimum of 80%HP. They seem a little incongruous, where sometimes below 80% is okay, sometimes not, sometimes with one engine at 80%... Would anyone have a better understanding to help me out? I must be missing something here haha!
On descent,
"At FL 250, reduce to idle thrust and adjust the rate of descent to maintain an IAS stated in the descent schedules"And then later on under 'Initial Approach',
"Whenever the aircraft levels off, initially set 85% HP RPM and adjust the throttle as required. Maintain a minimum
of 80% HP RPM until landing is assured."And elsewhere:
"Make use of speedbrake rather than reducing throttle below 80% HP RPM.""Protection against ‘design maximum continuous’ ice conditions is provided down to engine speed. During
descent, reduced anti-icing is available to a minimum of 75% HP RPM. With only one engine operative in icing conditions, up to 80% of the ‘design maximum continuous’ can be coped
with in climb and level flight.""For proper cabin cooling below 15,000 ft at high ambient temperatures, bleed air pressure should not fall below
30 PSI for extended periods. Pressure can be maintained by increasing the minimum engine RPM of one engine
to 80%." -
@buggsy44 Welcome to the world of vague, overly complicated, and sometimes contradicting 1960s aircraft manuals! ;)
For a descent in normal conditions, with both engines running and no icing conditions present, there are no rpm limitations on the engines. The normal procedure in that situation would be to reduce the throttles to 82% HP rpm until you are below 25,000ft and then you can reduce the throttles to flight idle.
In icing conditions with anti-icing switched on, the engines need to be at a high power setting in order for sufficient air to be tapped from the engines to provide heating to the airfoil and engines. So in order to provide enough anti-icing the engines have to be kept above 75% HP rpm. If the engines are reduced below this speed, the undercarriage warning horn will sound and will need to be silenced by pushing the button on the pedestal. As 75% HP rpm is a high power setting for a descent, you will need to use the speed brake to control the rate of descent.
If you were only flying on one engine in icing conditions, one engine is sufficient to provide a minimum of 80% of the normal anti-icing capacity of an aircraft with two functioning engines.
All of the above is also valid during the approach phase of flight, but another factor that creeps in here is the slow spool-up times of early turbojet engines. The engines take significantly longer to advance to full power from idle compared to if they were set to 80% HP rpm. Therefore during the approach and landing, it is recommended that the engines are kept at 80% HP rpm in case you need to add power for a missed approach, and you would use the speed brakes to control your speed.
On top of all that, you've also got the requirements to keep the bleed air above 30 PSI which requires the engines to be at a high power setting, or for the APU to be turned on (if it is within it's operational limits).
Hopefully, that helps explain it? It's one of those things that modern pilots don't really have to think about in normal operations with the advances of computers and the introduction of turbofan engines.
Mark - Just Flight
