Brake system analysis

Aircraft brake system

The objective of this lab session, was to provide an in depth analysis of an aircraft’s braking system. More specifically the braking system of the university’s harrier was under scrutiny.   To begin the session my group and I were shown a simple schematics diagram highlighting how the braking system works and the various parts that work in order to stop the aircrafts horizontal movement whilst on the ground. The system essentially works, through an assortment of hydraulic mechanical systems that are linked to the pilots pedal inputs (image1 (Coventry University 2016)).

 The systems involved, essentially then work to provide a force that squeezes the plates of the braking cylinder together therefore, slowing down the aircraft. In accordance with this, the harrier aircraft is also fitted with an anti-skid system. This system works to keep the wheels of the aircraft rotating when the brakes are applied. This is an important system for the aircraft as it helps manage the possibility of a tyre blowout, specifically if a wheel locked up during landing. After establishing the importance of the anti-skid mechanism, we were then provided with a scenario of a pilot suggesting that he feels one of his brakes are dragging but, he is unsure of which side of the aircraft it is.  In response to this scenario, we established the potential issues with an aircraft having brakes that are either unresponsive when released or when pressed. Obviously with reference to the instance of an aircraft with unresponsive brakes when the pedal is inputted, the danger would be the inability of the harrier pilot to stop when conducting a horizontal landing.  In the instance of the brake system not disengaging after application, many concerns can be raised. First of all the if the system is not releasing the brakes, and the pilot is not aware during a horizontal take-off, a full reheat take-off (in jets fitted with afterburner) could lead to a tyre blowout due to the high heat produced through the friction of the brake cylinders. In addition to that, this form of take-off would also be more inefficient than a conventional take-off as the aircraft would need to produce more thrust to move the aircraft forwards.  To correct such an issue it was decided that the best course of action would be to allow the pilot to gently apply the throttle control on the aircraft, (equally on a dual engine aircraft) to see which side the aircraft would lean to. After finding out which side was problematic, the next course of action would be to wait until the brakes have cooled down as they would most probably be at operating temperature. After the brakes have cooled the suggested method to rectify the issue would be devised by using an elimination process. As the issue would only be affecting the one side of the aircraft it would be assumed that anything connected to the parallel component e.g. the right or left brake input would not be at fault. This therefore, would narrow down the options a maintenance engineer would need to evaluate before choosing which part of the braking system needed to be analysed and/or replaced.

Procedure

·         Moving onto the main practical session, it was imperative to analyse the landing gear, and to see how best fit it was to remove the braking system. With regards to time management, the brakes were already removed as it is a strenuous task to remove the disc during the time we had in the session. Although the brake was already removed, we still removed the tyre to envision the removal process.

·         As the aircraft had not been operational for some time, the hydraulic fluid was not present in the aircraft and was therefore, liable to be jacked up by a hydraulic jack. This removal was done to the left wheel on the right hand side of the aircraft.

Image 2 Stator plate

•          The next stage of removal was with the use of a spanner to remove the central washer from the landing gear. There was no need to remove the surrounding bolts on the tyre as the rim was manufactured to be bolted onto the tyre, therefore making removal of these components strenuous and time consuming.
•          After removing the wheel the positioning of the brakes was highlighted, and so we moved on to analyse the already extracted brake disc.

•          The brake was comprised of many different plates that lay over each other, from this the plates were sectioned as the stator plate (primary plate) (image 2,Coventry University 2016) then a few segmented plates known as rotors (image 3, Coventry University 2016) up until the final torque plate (image 4,Coventry University 2016).  

Image 4 - Torque plate

•          After inspecting the brake plates, inspection of the torque plate and the torque tube came next. Housed inside the torque plate is the operating cylinder, inspection of this and the pressure plate was also carried out.
•          With inspection complete the tyre was then put back onto the landing gear. This was seen as a difficult task as the rim inside the tyre had to fit into a specific part of the landing gear frame.

Conclusions and implications  

From This lab session insight on the removal of a brake disc was given. With reference to the task however, assumptions had to be made about the positioning of the brake disc on the wheel, as the disc was already removed, to stay in line with time constraints. Bearing this in mind, this can be regarded as the only negative factor with regards to the session. On a more positive reflection of the session, knowledge was also provided on potential causes of failure for the brake disc, and explanations on why the discs were designed in the manner that they were designed in. With inference to the design of the brake disc, it was stated that the design of the rotor discs, was made to with heat management in mind.  The segmented design of the disc is imperative for the temperatures that the brake disc can experience due to frictional resistive forces. With the segmented design it means that, the disc would not be warped due to the metal being susceptible to expansion at high temperatures.  Further to this knowledge, as part of the lab session a carbon brake disc was also analysed to differentiate the advantages and disadvantages between the two disc materials.  Carbon advantageously being the lighter material for use on the brake disc was discussed as the successor to the steel braking disc. In addition to this, the carbon brakes have better heat resistant capabilities due to the macromolecular structure of the element. To the detriment of the carbon brakes however, the cost of implementing this material in the brakes is quite high so that was seen to be a disadvantage.  In addition to that, in real life application, implementation of carbon brakes into the harrier would not have been possible, as the they had not been designed yet during the Harrier’s manufacture, this would therefore suggest that if the aircraft was still in service, this would be a suitable way of improving the aircrafts performance.   

With reference to the learning objectives that were issued before the task was completed, I would suggest that I was able to effectively work with a team, to find possible solutions given a fault to the system analysed. In addition to that, as stated earlier, my ability to utilise all relevant information given during a crisis was further improved during the session, so as a whole I believe the learning objectives were met.