A healthy engine is vital when it comes to safe and reliable aviating. And a good overhaul by a reputable organisation has the added bonus of adding value to the aircraft. Here’s how to research thoroughly and spend wisely…
Jerry Parr
29 September 2021
The idea of having to have an engine overhauled can bring owners out in a nervous sweat. An avionics upgrade or a respray costs are more readily accepted because they give something tangible, but an engine overhaul is often regarded as one of those necessary evils – especially if the aircraft is essential to a business.
That said, engine performance decay is a slow process. Decreased performance tends not to be noticed until a newly overhauled unit is installed, then one realises what full power should really mean. In addition, an overhauled engine directly affects the value of the aircraft as a whole.
Let’s start with what is meant by ‘engine life’. The engine manufacturer recommends how long an engine can be operated before it is due for overhaul, i.e. its ‘life’.
Time Before Overhaul (TBO) is the term given to this ‘manufacturer-given life’ for an engine and is normally quoted in both engine hours and calendar time.
I should mention that not all engines are created equal when it comes to TBO. The majority of Lycoming engines are lifted at 2,000 hours or 12 years, whichever occurs first. Lycoming engines that live a hard life, such as in aerobatic machines or crop-dusters, are often restricted to 1,400 hours, whereas the Cessna 152/Piper PA-38 O-235 has a recommended TBO of 2,400 hours. Continental Motors’ recommended engine hours vary as well, depending on the model. And the later Rotax 912 has a TBO of 2,000 hours or 15 years.
On UK-registered non-EASA certified aircraft, once an engine reaches the engine manufacturer’s recommended TBO, it may continue in service.
“ An engine can continue to remain in service for private use only (termed ‘on condition’), as long as it undergoes regular checks in accordance with GR No 24”
The rules and regulations covering this period in ‘extension’ are found in the CAA’s CAP 747 Mandatory Requirements for Airworthiness under Generic Requirements No 24. If the aircraft is being used for training or private hire, then it is allowed to continue for a further 20% of its allowable life (known as running ‘on extension’), so for a 2,000 hour/12-year engine that would mean 2,400 hours or 14.4 years. Beyond this, the engine can continue to remain in service for private use only (termed ‘on condition’), as long as the engine undergoes the regular checks in accordance with GR No 24.
The GR No 24 requirements are not particularly onerous – cylinder compression checks every 100 hours combined with normal physical inspections and a continuous monitoring of oil consumption and leaks. At the moment, there is no allowable extension period for the Rotax 91x series of engines (or for some other engine types), although after 25 years of operation, I would imagine they should have amassed enough data by now to allow it – if anyone asks the question.
A number of years ago, GR No 24 was reviewed and a clause added that meant if an engine that was operating beyond TBO was stripped down for any particular reason (such as a shock load) then it could not be released back into service without being overhauled. Fortunately, the CAA changed this pretty draconian ruling to one that the engine’s condition must be assessed if undergoing heavy maintenance and be overhauled if required.
The overhaul requirement goalposts have now moved again following Brexit. The CAP 747 GR No 24 alleviations are still allowed for non Part-21 aircraft (as above) but can no longer be applied to Part 21 (EASA) certified aircraft. The options here now come under the Part-ML rules where an owner can stipulate in the aircraft’s ‘Aircraft Maintenance Programme’ the operating allowances when it comes to non-mandatory lifed items such as a piston engine. Airworthiness Directives must still be complied with and if the Chapter 4 of an aircraft’s (manufacturer-issued) maintenance manual applies any limitations to the engine hours or calendar time between overhauls, these must be adhered to.
This deviation from the manufacturer’s recommended TBO should be justified by the owner and whilst there are decades of evidence in operating engines to TBO +20%, to operate an aircraft in the ‘commercial’ (i.e. flying school) environment might be considered by some to be a step into the unknown and one would be wise to proceed with caution. There have been rumours that insurance companies may not cover aircraft with engines running ‘on condition’ for such use.
There are a number of options available when it comes to replacing a ‘life-ex’ engine – possibly more choices than usual when it comes to aircraft. The first decision to make is whether to overhaul or replace with new.
1. Buy new
Historically, GA aircraft engines are not replaced with new as the financial cost cannot be justified for it means a much larger capital outlay with little technical benefit. One of the exceptions to this rule is the Rotax 91x series of engines. The reasons are twofold. First, the overhaul cost versus the cost to replace with new is much closer than with the ‘standard’ aircraft engine. The reason for this is the components that Rotax mandate are replaced with new at overhaul – including the crankshaft. The second reason is that even a 91x engine at TBO still commands a decent price when sold as they are known to carry on quite happily beyond TBO in microlights and other non-certified aircraft. This means that for a Rotax, it may well be that the sensible option is to buy a new (and therefore latest specification) engine. That said, if the owner really wants it then the existing engine can be overhauled.
To replace a run-out Lycoming or Continental Motors engine with a new one does not make any financial sense. This then takes an owner down a different route of options available from either the manufacturer or an approved engine overhaul shop, namely overhaul the existing engine, purchase an overhauled engine, or part-exchanging the existing run-out engine (known as the ‘core’).
2. Exchange
When part-exchanging the core engine, it is expected that the crankshaft and crankcase are serviceable and can be used to build another overhauled engine. If either are not deemed serviceable, then a portion of the core charge (the maximum value of the part-exchange engine) will
be retained.
Normally, the original engine manufacturer will have overhauled engines that have been sitting on the shelf and ready for exchange. It may be for some of the rarer types that there is a wait while a replacement engine is prepared. An advantage of this is that the newly overhauled engine can be obtained in advance, sitting on the hangar floor ready to be installed as soon as the life-ex engine is removed. Downtime is therefore minimised, which is great news for a commercial operator who doesn’t make money when an aircraft is sitting in the hangar undergoing maintenance.
Some of the larger UK-based engine overhaulers may have examples of the popular engine models (such as installed in the Cessna 152 or Piper PA-28-161 Warrior) ready to ship, but this is not always
the case.
Realistically, no one is going to ship an engine back to the manufacturer to have it overhauled from the UK, so if one chooses the new engine to be supplied by the manufacturer, then it will be an exchange unit. One advantage of this is that the unit will be built to the latest specifications. It has to be said that there have not been that many changes to GA aircraft engines over the years – one relatively recent one was the introduction by Lycoming of roller tappets to help reduce camshaft wear. To incorporate roller tappets means changes to the crankcase so would not normally be incorporated by an overhaul away from the factory.
The engine manufacturers offer various ‘standards’ of overhaul. For instance, Lycoming offer ‘zero-timed rebuilt’ and ‘factory overhauled’ engines. Continental Motors offers ‘factory rebuilt’ engines.
3. Overhaul existing unit
There is a train of thought of ‘better the devil you know’ when it comes to having an existing engine overhauled and refitted to the airframe.
There is a lot to be said for this but quite often, the parts count that remain in use may be quite small. It is quite common for the crankcase halves to fret. This is where they literally rub against each other, wearing the metal away. Obviously, this can lead to issues with the alignment and running of other components. The situation is not normally fatal to the crankcase but invariably means shipping it back to a specialist shop in the USA for repair, although many UK overhaulers keep serviceable or repaired cases in stock to minimise downtime. Crankshafts also suffer through wear and corrosion and occasionally have to be replaced.
In decades gone by, it was common for engine cylinders to be overhauled. By and large, this is no longer carried out. The cost to overhaul a cylinder is so close to the price of a brand new assembly it means that there is no real value in refitting overhauled cylinders – one of the most stressed components in the engine.
There are a lot of Parts Manufacturer Approval (PMA) parts available for engines these days – from gaskets to complete cylinder assemblies. In the car world, these would be termed ‘pattern parts’. To produce PMA parts, the company must have been granted that approval by the FAA.
While the PMA parts may be cheaper than factory manufactured parts and are fully approved, they may not be the better option. It has to be said that there are far more Airworthiness Directives (AD) for PMA cylinder assemblies than those produced by Lycoming, for instance. Recently, some Superior Air Parts crankshafts have been the subject of an FAA-issued AD. That saga has dragged on for many months with some owners having to resort to sourcing a replacement crankshaft from another manufacturer as SAP are not producing new ones.
There are a number of approved engine overhaulers in the UK (and of course abroad). It pays to do the research, as with anything, but this is especially so when you are paying out an immense amount of money to have an engine overhauled.
There are pros and cons to looking in the UK or going abroad. Prices and reputation may both come into play with foreign shops, but when the shipping costs are taken into consideration and any potential warranty issues are considered, the dividing lines become closer.
The internet provides a wealth of information but should not be the only source of research. True, most engine overhaulers are not situated on airfield sites, so it means getting in the car (rather than an excuse to go flying) and personally visit the shops and see what they offer for the money.
Check on the background of the company – there is little point in having a warranty in place if the company changes its identity every six months. Who is the engine-experienced member of staff in overall charge? In the old days, these were the ‘D’ licensed engineer, a rating lost in the transition to EASA where the engine overhaul approval requirement is for the company to hold a 145 approval. It still needs someone at the top who knows their stuff when it comes to engine overhaul and operation. These days there are some non-145 approved companies who hold the approval to overhaul engines.
“Other components, like magnetos may need to be replaced at the same time…”
Normally, the aircraft’s maintenance organisation would also be consulted for its opinion on the options available – factory, field overhauler, and indeed which shop to go to. Bear in mind that it is not unknown for organisations to offer other organisations sweeteners for them to send them work. Listen to valid reasons based on experience from your maintenance organisation but remember it is your money.
When budgeting for the overhaul, it must be remembered that other components may need to be replaced at the same time, adding to the expense. Magnetos, starter motors and other accessories are often included in the overhaul quote but a propeller governor may be quoted separately. Worn or broken parts may be discovered during the engine removal (broken cooling baffles for instance) but some components are specified in the aircraft’s maintenance manual as a mandatory replacement at engine overhaul. These might be fuel and oil hoses, engine mounts and even engine controls.
While the engine is out, it is also the ideal time to really inspect the engine mount, replace the attachment bolts and ideally strip and respray the mount in white or light grey – cracks are inherently black and are much easier to spot if the engine mount isn’t painted black. Powder coating is not a good idea as the relatively thick coating can hide cracks.
In the UK, engine overhaul organisations are approved by the CAA to a standard. That does not mean to say that some don’t set their standards even higher. The factory might find it acceptable to build an engine and then paint it all over in one hit.
Personally, I think that an organisation that carefully builds an engine that looks like a work of art, with individually-painted components bolted together with new and shiny nuts and bolts, speaks volumes about its attention to detail.
When an engine is overhauled, it is released to service to run to TBO again. For instance, forgetting for the moment the complexities of extensions and running on condition, an O-320-D3G engine reaches 2,000 hours and is then overhauled.
This means that the engine is released to run another 2,000 hours (or 12 years). But, as far as its personal history goes, the engine has run 2,000 hours and the future hours are effectively added to that previous total. The second time it reaches TBO, the engine will have amassed 4,000 hours.
For an engine to be ‘zero timed’ (the common term for an overhaul), the engine would have to be returned to the factory and be overhauled (in Lycoming-speak) in accordance with their ‘zero time rebuild’ programme.
What should be noted is that any engine coming out of the factory, invariably has more ‘used’ components in it than an engine overhauled by field overhaulers. The engine manufacturers dictate what components must be replaced at overhaul (be it by the factory or elsewhere); whereas most if not all UK-based overhaulers would replace a camshaft at overhaul, the factory may not. Obviously, the components installed must meet the required wear limits but that does not mean they are new.
