The usual hydraulic fluid, Skydrol, is poisonous. If it makes it to the air conditioning system, it causes fume events that may have adverse health effects up to and including at least one fatality. And it is not easy to completely prevent the possibility of it creeping into the air ducts as bleed air is also used for pressurizing the hydraulic reservoirs (at least on some aircraft).

So why was this organophosphate-based fluid selected over anything safer? Are there no safer alternatives?

Update: I should have linked the Qantas A332 at Sydney on Dec 15th 2019, hydraulic leak prompts evacuation accident, which actually inspired the question. That one is clearly hydraulic-related (while in most other fume events we never know for sure), and there is a comment down the page:

By geezer on Monday, Dec 16th 2019 14:48Z:

Nothing to do with the apu other than the duct by the packs. Hydraulic reservoirs are (gas) pressurised by either an engine bleed or from the pneumatic duct in the pack bay. Hence a tee piece. If one system loses pressurisation, say overnight, then with a couple of leaky check valves one reservoir can slowly fill another back through the hydraulic service manifold, as all three systems are serviced from one inlet using a three-way valve. Eventually one tank is overfull to the point where the fluid now travels back down the air pressurisation pipe (a small bore pipe). Another check valve stops it flowing back to the engine so it ends up dribbling into the pneumatic duct in the pack bay. We know this because we have found skydrol in these small bore pressurisation lines. I hope you get all that. I think I need a little lie down now.

It should be noted that this is not verified and may not be the cause, but A330 has APU intake on the bottom, so the APU could have also ingested the hydraulic fluid dripping from the bottom of the fuselage into the inlet.

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    $\begingroup$ A big reason is that it isn't very flammable compared to other hydraulic fluids. $\endgroup$
    – Ron Beyer
    Jan 11, 2020 at 23:55
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    $\begingroup$ Yes 5606 is very flammable, with a really low flash point. Phosphate Ester is awful stuff to be around, one of the things, besides shift work, that got me out of mechanic-ing and up into the office back in the 70s. You have to make sure you wash your hands several times before going for a wiz. $\endgroup$
    – John K
    Jan 12, 2020 at 3:15
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    $\begingroup$ @JohnK That fluid seems to have a lot in common with hot sauce. $\endgroup$
    – PerlDuck
    Jan 13, 2020 at 10:54

3 Answers 3


Fume events are from two sources in the vast majority of cases: Deicing fluid, and engine oil.

The Aviation Herald event linked was poisoning from (really severe) engine oil contamination (mainly the toxic TCP additive; it's the same stuff used as a lead scavenging agent for running 100LL fuel on low octane engines, and it's also the oil additive that Lycoming specifies for the H2AD piston engines for the cam spalling problem). Phosphate Ester hydraulic fluid had nothing to do with it.

The oil contamination comes from 3 sources:

  1. A failed compressor section seal on the main engine at some forward bearing that is able to feed oil into the compressor upstream of the bleed port used for air conditioning.
  2. A blown seal on an air cycle machine (the turbocharger-like cooling unit in an air conditioning pack) allowing ACM lube oil into the conditioned bleed flow.
  3. An APU compressor seal failure (more or less the same as an engine oil seal failure) getting into the APU bleed and into the A/C packs.

Probably the majority of fume events are from oil traces, only enough to make smells, but which is really hard to remove, and deicing fluid (when airplanes get deiced and too much was sprayed into the engine and the engine didn't get enough of a purge run to clean the compressor out before turning on the bleeds). Deicing fluid fume events happen a lot and are probably the most common cause of fume turn-backs.

Skydrol/Hyjet is mostly a hazard to the techs that work on the planes. Anyone here who turns wrenches on transport airplanes will tell you that Phosphate Ester fluid is nasty, nasty stuff to get on your skin and probably the single most unpleasant aspect of the job.

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    $\begingroup$ damned knowledgeable answer! $\endgroup$
    – Fattie
    Jan 12, 2020 at 21:02
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    $\begingroup$ Is Phosphate Ester the reason I was told in avionics class to always wash your hands BEFORE going to the restroom? I was told everything is covered in a clear, thin film that feels like...burning... $\endgroup$
    – DKNguyen
    Jan 12, 2020 at 22:55
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    $\begingroup$ @DKNguyen yes. See my comment on the question post. On somewhere like a rear spar, or in a landing gear bay where you have hydraulic actuators and a lot of turbulent air, if any fluid seeps out of dynamic seals of the acuator it gets distributed everywhere you have a greasy film of it on everything, $\endgroup$
    – John K
    Jan 12, 2020 at 23:21
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    $\begingroup$ @UnrecognizedFallingObject Thanks But...TCP has been around forever. Developed for the B-36 IIRC alcorinc.com/wp-content/uploads/2012/06/… $\endgroup$
    – John K
    Jan 13, 2020 at 2:08
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    $\begingroup$ No, as a lead scavenging agent (ALCOR) it's been around for decades and that's what it was originally developed for. It was discovered it had anti-scuffing properties when added to oil later (creates a sacrificial oxide layer). The Lycoming TCP product is PN LW-16702 in a little clear bottle. It was always some mystery liquid and I found out it was TCP only a few years ago. $\endgroup$
    – John K
    Jan 13, 2020 at 2:14

Skydrol was developed to provide a hydraulic fluid that does not catch fire when a fine mist of particles under pressure is released, upon a leak in the hydraulic system. Of the two evils, poisoning or burning, a case can be made to avoid the latter when considering aeroplanes.

For ground based systems like full flight simulators, hydraulic systems are on the way out and only found in older sims. These often do not use Skydrol due to the toxicity of it, but old fashioned flammable oils like Shell Tellus. And then it makes sense to have the fire drills and extinguishers in place, although a simulator building does not have potential open flame sources such as a jet engine.

There is possible interaction between the hydraulic systems, the bleed air systems and the air conditioning systems. For instance for the B737, from this site:

The hydraulic reservoirs are pressurised from the pneumatic manifold to ensure a positive flow of fluid reaches the pumps... The latest 737's (mid 2003 onwards) have had their hydraulic reservoir pressurisation system extensively modified to fix two in-service problems 1) hydraulic vapours in the flight deck caused by hydraulic fluid leaking up the reservoir pressurisation line back to the pneumatic manifold giving hydraulic fumes in the air-conditioning ...

It should be possible to develop a hydraulic fluid that is neither toxic nor flammable, but the civil aircraft industry does not change practice until proven necessary and safe - for good reason. New developments are risky, and must be proven long term first.

Market forces and accident rates have proven to be insufficient for developing a non-toxic, non-flammable hydraulic fluid in the last couple of decades. Perhaps another tragic major disaster needs to happen first?

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    $\begingroup$ "although a simulator building does not have potential open flame sources such as a jet engine." Nor do they have the kinetic energy of a half-million-pound airplane moving at 150+ mph that can be dissipated in undesirable ways if contact with the surface doesn't go quite as planned. $\endgroup$
    – reirab
    Jan 13, 2020 at 9:40
  • $\begingroup$ Phosphate-ester hydraulic fluids do catch fire readily if exposed to open flame, albeit not quite so enthusiastically as the older alkane-based oils. $\endgroup$
    – Vikki
    Apr 22, 2020 at 20:07
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    $\begingroup$ FYI, MIL-PRF-87257 is a better fluid all around the Skydrol, from a flammability, health, corrosion, and hydraulic sealing point of view, and it what replaced 5606 in much of the DoD fleet (83282 is another option, but has a higher minimum temperature). But switching to it in commercial would be extremely expensive because phosphate-ester and petroleum fluids need different seal materials that are not at all compatible with each other. So no one will retrofit existing aircraft, and if a new aircraft came along you could share ground equipment between the two types. $\endgroup$
    – OSUZorba
    Apr 3, 2021 at 4:32

Organophosphorates reinforce Acetylcholine neurotransmitter's actions, by blocking its degradation, it have two toxicities: early and late, early is due to its action on neurotransmitters, late is induced by neuron lesions, and harder to treat. It is not difficult treating organophosphorate acute poisoning, as long as causative agent is clearly identified, and therapy started soon; in cases of pesticide poisoning, sometimes peasants know little about the features of product. Aviation Products with a deadly record in poisoning are Methanol, absorbed through skin and breath, Harry Ricardo lost and apprentice from this, and Ethylenglycol, used in cooling liquids to prevent it freezing. Propylenglycol is not that dangerous, reasons why its use is almost unknown in this field remain a mistery to me, perhaps production costs are much higher. Salut +

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    $\begingroup$ This seems to address the mechanism by which the chemicals are poisonous, but the question is why aviation fluids include them rather than something safer. $\endgroup$
    – fooot
    Jan 15, 2020 at 6:12
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    $\begingroup$ It's pertinent information though, as in that the poisoning effects can be managed and are well known. Another input into the risk management matrix. $\endgroup$
    – Koyovis
    Jan 15, 2020 at 7:32

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