Why 787 RAT Timing Matters in the AI171 Investigation
The Ram Air Turbine, or RAT, is an element of equipment that carries an enormous responsibility concerning safety of passengers on the board of transport aircrafts. For Boeing 787s, it serves as a back-up power source, when all main electricity, hydraulic or engine power sources get interrupted. That is why the disagreement concerning the timing of deployment of this particular feature on board of Air India Flight AI171 is more serious than a discussion about chronology of events.
According to preliminary results released by India’s Aircraft Accident Investigation Bureau, the activation of the RAT happened after switching off moving fuel control of the plane. On the contrary, the Federal of Indian Pilots believes that simulator tests show that it is not true. Therefore, it calls for conducting additional tests, because changes in sequence may mean that the RAT was triggered because of an electrical system anomaly.
By design, the RAT is supposed to activate only in special situations, but not during regular flights. It is a small wind-driven turbine, capable of providing either hydraulic power or electricity. Thus, once triggered, the device is expected to generate enough energy for maintaining such systems, as flight control, navigation, and communication of the airplane. In case of 787 models, deployment can occur both automatically due to loss of power and manually by pilot.
That is why the knowledge of the exact timing of RAT activation and generating enough power can become quite useful in the process of analyzing the development of accidents. As it was mentioned earlier, emergency systems of today’s airliners are integrated with flight controls, power, hydraulic systems, and indicators. Knowing when it was triggered and how much time was spent on delivering energy becomes an important aspect in understanding the problem.
Therefore, the focus of attention becomes timing. According to the pilots’ union, based on test results conducted in the simulator, deployment happens 18 seconds after the fuel switch operations. However, preliminary report says that the timing of this particular moment falls to be around 4-5 seconds. In light of these differences, they ask to conduct additional tests as proof of their claims. Besides, the pilots call for investigating the relationship between RAT deployment, generation of hydraulic power, fuel control switch, and acceleration measurements.
It is especially worth noting in regard to the fact that RAT is not only an emergency generator. This element of an airliner constitutes an integral part of the entire system of emergencies. The investigators will pay attention to such aspects as deployment time, the duration of its rotation, and the beginning of energy delivery to loads. These events are related ones, but it is necessary to investigate them carefully.
Besides the technological issue, there is a context of certification and airworthiness issues. The FAA issued its airworthiness directive for certain models of Boeing 787-8 concerning possible malfunction of the RAT during deployment at low air speeds. Those who are interested can find additional information on this matter in the FAA notice. It may help understand what makes this feature an engineering task, but it is not proven in regard to this incident.
Yet it is better to stop speculating at the current stage of investigation. Currently, there is no decision yet it is just a disagreement between the preliminary report and pilots’ claims. Additionally, there are many more factors which may become the subject of investigation along with flight recorder data, for example, cabin lights dimming, pre-flight message traffic, and others. In spite of being a part of the disagreement of the parties, it remains just a hypothesis of the investigation.
As it was mentioned above, the aircraft that departed from Ahmedabad Airport on June 12 was a Boeing 787-8 model. Unfortunately, the accident resulted in the death of all passengers, including people on the ground. But this is not the point of importance of this disagreement in this situation. The key feature here is that on highly-integrated airliners, a device used for generating back-up power serves as a system timestamp.
In case of confirmation of the sequence suggested by investigators, it will not be the only factor of the failure chain. However, if some corrections are required, it may change the way investigators think about availability of power and hydraulic system of the aircraft, as well as its cockpit system work during critical moments. Nevertheless, in any case, it becomes evident that on fly-by-wire airplanes, timing of auxiliary generator deployment cannot be overlooked.
By Thomas Caldwell — AMI’s senior editor for mechanical and mobility engineering, covering vehicle electronics, systems integration, electrification, chassis systems, propulsion, and safety policy.
