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. 

Metrology Lab

Metrology lab – Talyrond and shadow graph

For this lab analysis was put on the operating cylinder of the brake system. With further inference to this claim this lab was assigned to give an experience of a component inspection. Due to this, the first place to start was a visible inspection. From this inspection the piston housed in the cylinder was immediately found to be defective as it had NDT paint on its main surface. Due to the nature of the pistons movement during operation, (vertical displacement through the chamber) the metallic dye was understood to have rendered the part unusable, and therefore in need to replacement. The reasoning behind this was that during operation the piston needs to be smooth in order to slide through the operating cylinder without the risk of getting stuck. With the Paint on the surface of the piston this smoothness was compromised further to that, as the piston was housed tightly into the cylinder the risk of a malfunction improved drastically.  Continuing on with the visual inspection, the surface of the piston was also found to have various scratches on the surface in addition to that, the housing cylinder also has mild cases of deformation on the surface of the metal.  Although it was assumed that the operating cylinder was already unusable if this was a real maintenance task, for experience sake a metrology test was still conducted on the piston.

Procedure – Talyrond inspection (surface roundness)

·         The piston was removed from the operating cylinder as the cylinder could not fit into the talyrond machine. The piston was then placed into the central housing of the machine. When placing the piston in extra care had to be taken to make sure that the piston was centred and not leaning to one side to eliminate the risk of systematic error.

·         After placing the piston centrally into the apparatus, the jaws of the machine were then closed in simultaneously, this was also important to centre the piston.

·         The height of the needle was then adjusted to match the location of the most visible scratch on the piston.

·         Once locked and held into position the needle of the talyrond machine was then adjusted, using the thimble that gauged the distance between the needles head and the piston surface.

·         Looking at the distance visualised on the PC monitor, the distance between the needle and the component was set to a separation of 0 micro metres.

·          Once In contact with each other, the test was then simulated and the piston was rotated with the spindle of the talyrond machine gauging the deviation of the part, using the zero value as reference point.

·         After the doing roughly 3 revolutions on the component the results were then shown on the computer screen highlighting the deviation from a perfect circle reference generated on screen.  

·         This test was the simulated over 5 times at varying heights in order to surmise the components surface roundness. In addition to this, repeating the test was imperative to remove anomalous results, and so each time the test was started the component was re-centred therefore making the results more accurate.

Results

From the Talyrond inspection, results on the surface roundness of the piston were found. Evidence and analysis of these results may be highlighted from 4 key pictures that were taken during the test.

 

 

From these initial images of the test simulated at the same height as the scratch on the piston, the deformation of shape was clearly shown.  Using the perfect red circle generated by the software as the reference point, the green circle representing the surface of the piston is shown to be relatively round up until a certain point where there is a sharp dip shown. This dip highlighted the deformation that the scratch had placed on the piston as the ball of the machine lost contact with the cylinder. In addition to that, general expansion of the piston was also shown through these tests as the circle was shown to grow outside of the reference point at certain locations. This deformation could have been caused by the metal of the piston expanding after working at constant high temperatures. 

For this form of NDT the tip of the talyrond machine was ruby. The reasoning behind this was associated to the properties of Ruby. Primarily Ruby is considered a strong stone can easily be manipulated into a round shape. Bearing this in mind, this property allowed it to last for a long time, whilst being effective at its job. In addition to that, the property for it to be easily manipulated gave it the edge over diamond, as that would be more costly to cut and shape as diamond tools would need to be used to cut the stone.

Shadow graph

This form of NDT (non-destructive testing) was needed to inspect the elements of the brake operating cylinder. The reasoning behind this was that the cylinder was too big to fit into the jaws of the talyrond machine, this meant another form of testing had to be used to see if its surface was damaged.  The shadow graph was able to provide a more in depth visual of the cylinder, highlighting any cracks and deformations.

Procedure

·        The cylinder was placed on the top of the shadow graph machine and the machine was turned on producing a light that created a shadow projection of the cylinder.

Shadow graph machine

·        On the screen of the machine, the profile of the cylinder was then focused to give clearer view of the component.

·        By altering the positioning of the cylinder, varying angles of projections could be formulated essentially giving a more in depth view of the cylinder than the visual inspection.

Health and safety

Over the course of the lab sessions various precautions had to be taken with regards to safety. From the brake task the hazards were as follows;

• potential slip hazard from hydraulic fluid if the aircraft had been operational.
• possibility of hitting your head when working under the aircraft wing to remove the tyre.
• working with heavy loads e.g the tyre during removal. 
• possibility of working with hot materials is the aircraft had been operational and service began immediately after the aircraft landed.
• working alongside the sharp edges of the aircraft wings.

With respect to the following health and safety issues the following measures were/would (be) used to mitigate the hazards;

• As the aircraft had not been operational for a long time, the aircraft did not need to be drained of hydraulic fluid from its landing gear so that hazard was already dealt with. In addition to that, for the slip hazard, use of an absorbing agent to contain spills would be a suitable way of managing the concern.
• For this particular hazard in the context of lab the only mitigating factor that could be implemented is for the maintenance engineer would be to show precaution when standing and getting under the aircraft wing. In a real world scenario however, the aircraft could be hoisted up using various devices to eliminate that risk. With consequence to this however, the risk of working with heavy objects at height would then arise, which is more detrimental to a persons safety.
• Working as a team or using machinery designed to hoist the tyre out safely would be a mitigating factor for this hazard. With regards to this however, the tyre was not really that heavy when inflated so if deflated the task could easily be performed by a fit person.
• Use of a cooling agent such as water could be used to cool the brakes after the aircraft comes to a halt. Consequently to this approach however, if this was done multiple times the constant sudden change to the temperature of the brakes, would lead to their shelf life decreasing at a faster rate as cracks would form through the rapid expansion and contraction of the metal. so the most reasonable method of mitigating this risk would be to undertake other tasks on the aircraft in the meanwhile, whilst the breaks cooled of naturally.
• use of foam pads on the edges of  the wing, so that if someone hits their head the blow is cushioned.

For the metrology lab there were no real concerns with regards to health and safety. With regards to this claim I would suggest that i a person was competent enough to understand the machines he was using and showed reasonable precaution to the mains connection, there was no real danger to their health.

In addition to this however, in the case of another form of NDT a precaution when dyeing the component under scrutiny, would be the use of protective clothing such as gloves to stop the dye coming into contact with the skin, and the use of a facial mask to stop the fumes of the dye from being inhaled.