APUs • MAINTENANCE
THE POWER BEHIND THE THRONE
The first time many owners discover they have an auxiliary power unit (APU) on their aircraft is when it stops working. Which is why you may want to put it on an APU maintenance programme. Words Alasdair Whyte
Under the hood: APUs offer increasing reliability but have life-limited parts.
APUs • MAINTENANCE
Under the hood: APUs offer increasing reliability but have life-limited parts.
THE POWER BEHIND THE THRONE
The first time many owners discover they have an auxiliary power unit (APU) on their aircraft is when it stops working. Which is why you may want to put it on an APU maintenance programme. Words Alasdair Whyte
OWNERS have a similar relationship with their aircraft’s auxiliary power unit (APU) as middle-aged people do with their spine. For years you rarely think about it. But once you have a bad back you realise how much it does for you.
“Because they are not main propulsion, APUs are treated like an accessory,” says Cole Reed, director, Engine Services at JSSI, the industry’s largest independent provider of maintenance programmes. “When everything works, nobody thinks about it; the air conditioning runs on the ground, the electrical power runs on the ground, the engines start without a cart – but once there's an issue, all of a sudden it becomes a problem.”
Like any complicated machine, APUs can have mechanical issues. Aircraft can be grounded by APU problems, but there are many service centres able to install a line replacement unit (LRU) without removing the APU.
If the APU does have to be removed and replaced there is a decent supply of loaner APUs available – although this varies by type. TES, the Dallas maintenance and repair company, has more than 250 Honeywell 36-100 and 36-150 APUs (it bought Honeywell’s APU loaner fleet in 2021). Gulfstream also has a significant number of leased APUs and parts for Honeywell APUs. JSSI has a rental pool of more than 120 engines and APUs. Engine Assurance Program (EAP) also has loaner units.
APUs offer ever-increasing reliability, but they do have life-limited parts (LLPs) – particularly the compressors and turbine rotors – that will eventually need to be replaced. An APU overhaul can easily cost more than $500,000 with larger aircraft APU overhauls often above $1m. Costs can add up quickly. Removal and replacement (R&R) costs can easily be more than $30,000 on their own. These multiply if you are installing a rental APU.
“It is important to understand what kind of APU you have and the cycle or time limits on those APUs. Some may have a 10,000-hour limit for the turbine rotor and some might have a 4,000-cycle limit. Some older APUs have hot-section inspections where they are taken off the aircraft and torn down,” says Jack Tadvick, sales manager for APU programmes at Honeywell Aerospace. “Newer APUs like the RE220s are on condition but will have LLPs at 10,000 hours or 20,000 cycles.”
The ratio between hours and cycles varies by aircraft type. A large cabin aircraft typically operates at about one cycle per hour. However, a Citation Excel used by a charter company (which tries to stay on the ground for a short time between flights) could have 3.5 cycles for every hour the APU is used. For simplicity, hourly maintenance programmes charge by the hour and not the cycle.
Honeywell Aerospace makes a range of APUs for small up to large-cabin jets.
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Honeywell Aerospace’s HGT400 APU supplies pneumatic and electric power while the aircraft is on the ground or in flight.
Honeywell Aerospace’s HGT400 APU supplies pneumatic and electric power while the aircraft is on the ground or in flight.
Should you enrol on a maintenance programme?
The majority of APUs are enrolled in hourly maintenance programmes, but those numbers drop for older aircraft. Almost 90% of modern Dassault, Bombardier Globals and large Gulfstreams have APU maintenance programmes.
Banks and financial institutions are increasingly pushing their customers to enrol engines in maintenance programmes. They like knowing that maintenance is covered and that programmes can make aircraft easier to sell. A few financial institutions insist on APU programmes as well, but some of the larger banks remain more relaxed.
“Being on a programme doesn’t add value. But not being on one is a major issue. Given the high propensity for failure, an unenrolled APU can mean a deduction at valuation,” says Jason Zilberbrand, president, VREF the aircraft value guide and an ASA appraiser. “It's one of those unique situations where you don’t make any money by being on the programme, but you can lose a whole bunch of buyers by not being on it – and you’re theoretically handing a pile of risk to the next owner. Programmes don't earn a premium, but their absence costs you – in price, buyer pool and time on market.”
JSSI’s Reed agrees: “An APU programme can make a sale go faster, because the future maintenance risk lies with the programme provider rather than the buyer or seller. A fully protected aircraft often puts the seller in a stronger position on asking price, as the coverage can be transferred with the jet at sale.”
The main choice is whether you want to go with a manufacturer like Honeywell Aerospace’s Manufacturer Service Program (MSP) or Safran’s Powercare; or with an independent provider like JSSI or Engine Assurance Program (EAP). Bombardier also gives owners the option of adding APU coverage to its Elite Smart Services cost-per-flight-hour programme.
“Being on a programme doesn't add value. But not being on one is a major issue.”
Jason Zilberbrand is an accredited senior appraiser and president of VREF Aircraft Value Reference & Aircraft Appraisals.
Honeywell Aerospace offers two options: MSP and MSP Gold. Both cover major scheduled and unscheduled maintenance events, base parts exchanges and standard overhauls. MSP Gold includes the cost of removing and replacing APUs and the cost of shipping parts, LRUs or the whole APU.
“The reason most people go to MSP Gold is the logistical support for AOG on trips. If you’re AOG due to the APU then that covers a channel partner road crew to come out to you and it pays for all of their transportation and hotels and all that to come to you and fix you up and get you moving again,” says Honeywell Aerospace’s Tadvick. “If you are flying over the pond or to airports that do not have maintenance, you should definitely go with Gold.”
JSSI also offers different tiers of APU coverage. Its core programme covers scheduled and unscheduled shop maintenance, replacement of life-limited parts, rental APU costs and future service bulletins and airworthiness directives. The APU+ upgrade adds freight, labour for removal and replacement at scheduled and unscheduled events, labour for routine inspections and AOG logistical support, including travel expenses and expedited shipping. APU coverage can be bundled with JSSI's engine or airframe programmes or purchased separately.
Some new aircraft owners delay enrolling APUs because of warranties. But JSSI’s Reed says that could be a mistake. “It's easy to think you don't need coverage on a brand-new aircraft because you have the warranty. What often gets missed is that a programme covers things the warranty doesn't: removal and replacement labour, freight for the APU, rental costs while yours is in the shop. The coverage is complementary to the warranty.”
For an in-service aircraft that does not have APU coverage, it is possible to buy into a programme at any stage in the aircraft’s lifecycle. One APU specialist says that buyers should insist the cost of this is automatically deducted from the sale price when negotiating.
Reed argues that hourly cost programmes protect owners from maintenance cost inflation. “You're paying at today’s rates for an event that may not happen for years. When it comes due, the funds are already in place. It takes the escalation risk and the surprise out of what can be a six or seven-figure event.”
It also removes uncertainty for owners. He adds: “The biggest piece of advice I would give is don't forget about the APU. It powers critical systems on your aircraft and maintenance protection deserves consideration.”
The cost of transporting an APU can rise rapidly.
The cost of transporting an APU can rise rapidly.
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APUs for beginners
How many engines does a Gulfstream G700 or a Global 8000 have? If you said two, you were wrong. You forgot about the auxiliary power unit (APU) at the back.
Unlike the showy engines on the aircraft’s wings, APUs do not attract a lot of attention. This is partly because they are hidden in the tail cone at the back of larger aircraft (sometimes referred to as the dungeon by mechanics). They are deliberately placed as far from the door and cabin as possible to reduce noise.
When an aircraft is on the ground, APUs provide power to the cabin and cockpit – most noticeably for air conditioning – without having to turn on the engines. Larger ‘bleed’ engines need an APU or external power to start. APUs run on JetA1 from the main fuel tanks, while smaller aircraft rely on engines or plugging aircraft in to ground power trucks or cables.
An APU can restart an engine or provide power to the cabin and cockpit during flight. When captain Sully Sullenberger landed on the River Hudson, he was able to use a Honeywell Aerospace APU to provide power for avionics and control surfaces after both engines were taken out by birds.
The fact that it was a Honeywell Aerospace APU is no surprise. The company dominates the market. It has been building APUs since 1948 and delivered more than 100,000 of them. More than 150 commercial, regional, business jet and helicopter types use Honeywell Aerospace APUs (particularly the 36 Series).
In business aviation the 36-100/RE-100 and RE-100/36-150 APUs are used on the Bombardier Challenger 300, 350, 3500 and 600; the Dassault Falcon 7X, 8X, 2000 and 900; the Embraer Legacy 450 and 500; the Gulfstream G280, G450, G550, GIV, GV, G650 and G650ER; the Textron Aviation Citation XLS; and the Hawker 800, 800XP, 900 and 4000.
APUs are specific to aircraft types, but they can be interchangeable within a family. That means you can switch the same APU between a Challenger 300 and a Challenger 3500, for example, but not between a Falcon 7X and a Praetor.
Honeywell Aerospace has the larger RE220 APU which is used on Bombardier Global 6000, Challenger 850, Global Express, XRS, Global Express 5000, Gulfstream 650ER and Gulfstream G550 aircraft. The newest version of the RE200 is the HGT 400 on Gulfstream’s G500 and G600 and Falcon’s 10X.
Honeywell Aerospace has more than 90 APU authorised sales and service partners worldwide for its RE100 series and 77 for its RE220.
One of these is Gulfstream which has a large APU facility in its Savannah Service Centre in Georgia. Offering APU servicing alongside scheduled maintenance means that OEMs do not need to ship APUs to other facilities. This should save time and money. This is why Duncan Aviation has invested in heavy maintenance at its Lincoln, Nebraska APU shop. From early 2027 it will add full overhaul capabilities for Honeywell RE100 APUs on the: Learjet 45 and 75; the Citation 560XL, Citation XLS, Citation Sovereign and the Citation Latitude.
“Our customers already trust us with their aircraft and engines every day; this adds to that one-stop-shop experience for all things engine and APU,” says Brad Wales, engine overhaul manager, Duncan Aviation. “By bringing this heavy work in-house, we are providing our customers with the same quality and care they have come to expect from Duncan Aviation, while delivering a more seamless, end-to-end service experience.”
Honeywell Aerospace’s main challenger in business aviation is Safran. This business acquired two APU programmes from Pratt & Whitney in 2014. The SPU150(DA) is found on the Dassault Falcon 5X and the SPU300(BA) on Bombardier’s Global 7000 and 8000.
There is a lot of research going into reducing APU noise with the goal of creating electric APUs or eAPUs. Honeywell Aerospace’s APUs are already able to use a 50:50 SAF and Jet A1 blend. They will soon be able to use 100% SAF.
There’s a good supply of loaner APUs.
