PDM at work...real time.

Ok, so we’ve seen it all, and content that PDM can be useful. But how it works, is maybe better explained with an example.

An aircraft is on a flight between say Mumbai and Delhi. The sector for that aircraft for the day is something like this – Bombay, Delhi, Jaipur, Bhopal, Chennai, Bombay. Now I agree that’s quite a sector, but I need it to be that long to emphasize how and where PDM comes in.
Between Mumbai and Delhi, the Number 2 engine shows a slightly higher EGT (Exhaust Gat Temperature) than the number 1 engine. It’s still within limits, and therefore the pilot does not pay too much heed to it. However, EGT sensors still attach a signal of this to the ACARS and the AME on the ground at Delhi, receives this information. He makes a note of this in his inspection pocketbook which he will refer to when he’s making an inspection of the aircraft before signing out the aircraft from Delhi to Jaipur.

When the Aircraft arrives at Delhi, he does a quick examination of the number 2 engine, checking externally for anything unusual, checking oil levels, maybe a chip detector check and other routine inspections that could be indicative of the high EGT condition. He finds nothing unusual, but just puts a ‘tag’ on the ACARS note he received, so that the situation does not go unnoticed by the AME’s at the next destination.

Between Delhi and Bhopal, the situation is unchanged. However, a little after take off from Bhopal for Chennai, the EGT comes within the caution range. The pilot notices this and makes an entry in the PDR (Pilot Defect Report). However, even while the aircraft is in the air, the AME at Chennai, notices this via the report obtained from the ACARS. He also notices the small tag left by the AME at Delhi. He gets on the phone with the AME at Delhi to get a quick briefing on preliminary findings and the probable cause. Armed with some information he takes a look at the engine when it arrives at Chennai. He also has a quick word with the pilot on his version.

He decides to do an in-field/brief boroscopic examination; quite similar to an endoscopic examination done on humans to make a visual check of the state of things inside the engine. Ok, he’s found the likely cause of the rising EGT. There seems to be an oil leak at a bearing compartment which is likely due to partial damage to an oil seal. He makes an assessment of the extent of damage, and concludes that there is some cause for concern, but not something which would require grounding the aircraft at Chennai. So, he makes an entry in the engine log book, replenishes the oil, signs out the aircraft, and briefs the AME in Bombay about the situation. Aircraft leaves for Bombay, and while the EGT continues to hover in the caution range, it does not exceed the limit.

Aircraft lands at Bombay at 10p.m and has completed it’s sectors for the day. It’s next flight is due next day at 7a.m. Overnight, the AME with his team of specialised technicians (whom he’s assembled even before the Aircraft arrived in Bombay, having had the information about the problem in advance), rectifies the problem, and completes an engine test run. The Aircraft is ready to go at 0700hrs next day!

Now, that’s an example…! Let’s analyze this further next time!

A world Without PDM....

From an Aircraft Maintenance engineer’s perspective, it’s like walking to an aircraft that just arrived, blind! Not a clue about even the existence of the snag let alone the system affected or the location. Pilot defect reports are a help but they can sometimes be cryptic, and sometimes non-specific to the mechanic. This in turn requires further discussion between the pilot and mechanic to put a finger on the exact nature and cause of the problem. All this while…you’re eating away into that precious commodity – TIME!

Given the shortage of time then, and, the pressure to get the flight going (waiting passengers can get very irksome, even dangerous!) the AME usually ‘defers’ the defective component/ item. That is, within the framework of safety, defer the maintenance task to be conducted at the next base, and release the flight for now.
In plain terms, we gather information about the defect at Base # 1, and have it rectified at Base # 2; that is, ofcourse if the maintenance on the defective component can be deferred. If the component is an absolute necessary for safe flight,…well, irksome passengers…aka screaming, a little crying, lots of demanding…!

A maintenance world without PDM, is a reactive one at best. One that potentially compromises safety, increases costs, and leaves a lingering thought in the mind of the AME, “I hope…..!”

Next up, an example of PDM.

Predicting Aircraft Maintenance..?

Going by the cue of my last post, time, or more fittingly the conservation of it, appears to be the primary motivation behind Predictive Maintenance (PDM). So what exactly is PDM?

Imagine an aircraft soaring at 36000ft develops a snag say in it’s hydraulic system. Essential items that are affected by it include Flight control systems, Landing gear systems, Brakes and Anti-skid etc. Now, there are independent standby hydraulic systems in most airliners today. So, you wouldn’t exactly plunge earthward; but, you still need to ensure that the snag is rectified at the next base. I.e., the aircraft cannot take-off to it’s next destination without snag rectification. PDM systems basically detect the snag (in flight) and transfer detailed descriptions and locations of the snag to base via ACARS (more on ACARS here). The mechanic on duty gets the details, studies the snag, prepares a rectification action plan and gets all necessary equipment and manpower ready. Just as the aircraft lands and rolls in to the gates, the mechanic trundles out, fully equipped, to the aircraft. While passengers unload/reload, he has rectified the snag and the Aircraft is ready to go!
Simple? Yes, but there are constraints here too. More on those later. Right now, we know what a predictive maintenance can do, and, that it can prove to be an important advantage. One that puts time…on your side.