Passing The Smell Test

Just a few stray drops of oil in the environmental control system can stink up an aircraft cabin, but airlines can avoid this operational headache by switching to a new type of turbine oil, explains Katie Lutton, Technology Manager for Eastman Aviation Solutions.

How do oil odors enter the passenger cabin?

The most common cause is drops of oil that leak from the auxiliary power unit (APU), and then are drawn into environmental control system (ECS) and the bleed air supplied to the cabin. For example, when the aircraft moves, any oil drips on the fuselage travel to the back and then get sucked into the inlet duct of the APU, which is at the rear of the plane. Some aircraft are prone to this because they have that inlet duct on the bottom of the aircraft in a six o'clock position, whereas other aircraft models place the duct more towards the top of the aircraft. The drops of oil themselves are commonly caused by over-servicing – when they're filling the oil tank. If you get even a few drops on the exterior, drips can form along the body. Also, on the APU itself that fill point is only a few inches away from the inlet duct, so you're very likely to get any excess fluid sucked in and fed directly into the ECS. If this happens, just a few drops are enough to cause an odor event.

What do passengers experience in an oil odor event?

While oil odors aren’t hazardous, they can be very unpleasant for passengers. Within turbine oils, the polyol ester basestock reacts with heat and water over time to turn the ester into its original component, a fatty acid. So that's where the characteristic ‘dirty socks’ smell comes from. The interesting thing about these acids is that they are very small molecules, highly detectable and highly offensive by smell. Humans can detect one of these acids, isovaleric acid at as little as two parts per billion, and it smells like rancid cheese. Passengers tend to notice these oil odors most commonly during the ascent and then the descent. Used hydraulic fluid, meanwhile, can smell weirdly like banana and garlic!

Are oil odor events more likely on older aircraft?

Across the industry, there are around one to two odor events due to oil every 10,000 flight hours. But we’ve seen brand new aircraft, within their first 2,000 flight hours, having an oil odor event because of leaks into the environmental control system. It only takes one drop a minute to actually smell something in a cabin. Perhaps older equipment will have events due to older seals that aren't working as well, but in general there's not a huge correlation between age and frequency. What we do see is that if you have one odor event, you're more likely to have a second. And sometimes for airlines that's immediately after because those few drops of oil within a complicated environmental control system are really hard to find. If they don’t pick the drops up with their UV lights the maintenance report may say ‘no fault found’, and a few flights later they are getting the smells again.

What impact can the choice of turbine oil have on odor events?

Eastman’s latest turbine oil, ETO 2330, has an inoffensive odor profile, which helps address those one or two odor events per 10,000 flight hours. It smells more like old olive oil rather than dirty socks. In terms of the chemistry, by increasing the carbon chain and making bigger molecules, you simultaneously increase the threshold for humans to smell it and decrease the offensiveness of the smell.

And what about its impact on APU reliability?

Our new oil chemistry also enables greater wear resistance and load carrying. Cold starts of an APU at altitude can turn engine oil into the thickness of molasses, causing scuffing and wear on gear surfaces, but by changing the anti-wear chemistry we're able to better protect those surfaces during this critical time for the APU. We’ve also focused on avoiding excessive coke deposition, which can lead to failure of seals, oil leaks and then odor events.

What cost and time savings does the new oil offer operators?

Alongside less APU wear, the cost is less than mechanical solutions to odor events like VOC converters and advanced air filtration systems. These often require complicated installations or retrofits, whereas the new oil can also be adopted much more quickly, at just over an hour with our flush-fill procedure, and will have less ongoing maintenance required. We've also ensured that the new oil's compatible with all the latest in-service components and sensor technology, despite the new chemistry being a little different than other engine oils today. Finally, using a higher-performing oil means fewer unplanned maintenance events along with more on-time flights, which I’d say is the modern passenger’s number one priority.