Dear friends and followers, welcome back to my channel. Today's video covers another very important question: What is the RAT?
So let's get right to it!
{Intro}
Okay, get your notebook out.
RAT is short for Ram Air Turbine, and the RAT is one of the things you rarely see on an airplane.
So let's cover some basics first, and then we'll use an Airbus A320 as an example.
But the system in general is very common in most commercial jets.
Okay,
first to know is that the Airbus A320 has three independent hydraulic systems:
Green, blue, and yellow, and one of their highest priorities is to provide pressure to the
actuators to move the flight controls, such as the ailerons, the rudders, the horizontal stabilizers, and the flaps and slats.
Now two of those hydraulic systems, the green and the yellow, are pressured by engine-driven pumps connected to the main engines.
The third, the blue system, is
pressurized by an electric pump and besides normal operation, acts as a backup in an emergency event, but more about that in a minute.
Besides the fact that the main engines provide pressure to the hydraulic system, each engine has a generator
producing AC power to feed the aircraft
electrical system. As a backup, you have the auxiliary power unit (the APU)
which can power the entire system on ground or in flight, and another backup are the batteries which provide DC power,
which is then inverted to AC power for the necessary system supply.
But they will only provide power for a minimum of twenty minutes in flight. Now let's imagine this scenario:
You're in cruise flight and for some unknown reason, both engine-driven generators suddenly fail,
meaning the electrical power supply is immediately lost.
To make matters worse, you've taken off with an inoperative APU.
The batteries will take over, but as mentioned before, they will only last for twenty minutes. You can see where I'm going with this.
As both main generators fail, the main AC supply is lost and the plane turns into an emergency electric
configuration, and that's
where the ram air turbine comes to play.
The RAT Immediately drops out of the belly of the aircraft.
As you can see, it looks like a little windmill and that's more or less what it is. The blades
Immediately start spinning due to the airflow and powers a hydraulic pump, which sits in the RATS to pressure the blue
hydraulic system, because as I've mentioned before, the blue hydraulic system is normally powered by an electric pump,
but due to the emergency elec configuration,
the electric pump has now become inoperative. So some of the created hydraulic pressure is then guided into the
CSMG
(the Constant Speed Motor Generator),
which then provides enough power to feed the electrical system, but the output is only
1/18th of the normal supplied by the engine generators.
Therefore, a lot of the electrical consumers, like the passenger entertainment system, the cabin galley outlets (like the coffee makers, etc.), and
most of the cockpit instruments are then shed or shut down to the necessary
minimum. Now in case the RAT doesn't automatically deploy, there is a guarded push-button switch on the electrical panel
(located on the overhead panel) which allows you to manually extend the RAT at any given time. Or
as you can see in this video, it can be [extended for testing purposes.
During the extension of the RAT (which takes up to 3 to 5 seconds) the batteries shortly take over and
after the extension, the angle of attack of the RAT blades is
governed to maintain a constant pressure in the blue hydraulic system. For the RAT to function normally,
you need to maintain at least a speed of 140 knots.
So
after landing, the RAT stalls, and the batteries take over.
Okay, another scenario which most of you are familiar with:
The Hudson River landing by "Sully" Sullenberger.
After experiencing multiple bird strikes, they lost both engines.
Now, as mentioned before, the main hydraulic pressure is
created by the main engines, and the blue hydraulic system is pressured electrically.
In that situation, they had no hydraulic pressure nor electrical power, and
as many of you know, the Airbus relies on electrical power due to the fly-by-wire system, and
hydraulic pressure to move the flight controls. Again, the RAT immediately deployed, providing electrical power
but now even more importantly, pressures the blue hydraulic system so that they had control over the flight controls, and
safely landed the plane in the Hudson River. And here
you can see the housing of a RAT on the Airbus A380.
It's installed on the left wing flap fairing due to the fact that the airflow is
smoother than near the belly. And this one here shows the housing of the Boeing 767.
So as you can see, the ram air turbine is a vital part of the airplane in such a critical situation,
therefore making the airplane very redundant.
But a lot of things have to go wrong until the RAT deploys, and
there are only a handful official incidents where the ram air turbine recovered control over the plane.
I hope you enjoyed this basic introduction video of the ram air turbine. Make sure to tune in next Thursday,
swing by my Instagram account (the link is in the description below), and subscribe to my channel here and
spread the word. All the best, your Captain Joe.