Lock (in actuality, Cavitation) Mysteries Revealed
Below, in my humble opinion, is an excellent
narrative written by Bonanza owner, Jack Letts, who, in his day job is something
of an extraordinary pump expert. This narrative is a succinct explanation of the
technical aspects of what causes what we pilots have long called "Vapor Lock"
but in actuality is cavitation in the pump world.
First a video primer on "Cavitation" is in
Cavitation (aka Vapor Lock)
Mysteries Revealed by Jack Letts
Summer is upon us and the classic pilot complaints and reports of fluctuating
fuel flow have started. Last summer I wrote three long posts in
cavitation. Together those posts formed a primer on Cavitation and I think they
offer a nice background to help understand what's actually going on inside the
pump and fuel lines.
Cavitation is often referred to as Vapor Lock when discussing engine fuel
systems, but I don't like the term because "Lock" is misleading; it implies that
the system is locked and that no fuel is flowing. In fact, the system is working
just at diminished capacity, it's not locked.
First, let me expound a little on why the fuel flow falls off with altitude.
I've been in the "pump business" for 25 years so it's in my wheelhouse.
I think George
Braly's Socratic teaching method is great. Give a little hint then let the
student figure it out, in figuring it out the student gains a deeper
understanding. For those of you who would just like to have it explained to you
let me take a swing at it.
Pumps need atmospheric pressure in order to work. Pumps that draw a suction
lift, like the engine driven pumps on our beloved Continentals that must draw
the fuel up out of the low wing tanks in a Bonanza are even more dependent on
Picture when you are drinking through a straw. Most people think they are
"sucking" the beverage up into their mouth. In fact what is happening is that
you are lowering the pressure in the straw and atmospheric pressure is pushing
the tasty beverage up the straw. If you can picture somehow trying to drink
through a straw in a vacuum, you can easily see that it wouldn't work. No
differential pressure, between the surface of the liquid and the inside of the
straw would mean no fluid movement.
At sea level, standard atmospheric pressure is 14.7 PSI, expressed another
way that all us pilots know, 29.92 inches of mercury. If you want to keep
thinking of a column of water like in your straw it would be about 33 feet.
As you climb, obviously you have less atmospheric pressure. By the time you
get to 15,000' you are down to about 1/2 the sea level pressure. So what
happens, why is the pump now putting out less flow at the same pressure? In a
Cavitation is a word that is used regularly but I find that it's poorly
understood. To explain cavitation I like to use the swimming pool example.
Imagine you are standing in the shallow end of a swimming pool and you take your
hand and you move it as fast as you can just under the surface. You'll see a
whole stream of bubbles appear behind your hand and they will immediately
disappear as soon as you stop moving your hand. Your hand did not break the
surface so the bubbles did not come from the atmosphere. Where did the bubbles
go? they didn't float up and break the surface like air bubble, what are they?
What you have done is put more energy into the water than it has affinity for
itself. You've created little vacuum, or more accurately steam bubbles that slam
back shut as soon as the energy that created them stops.
Now here's the next interesting observation. If you put on your scuba gear
and dive down to 33' feet, you can't create those bubble that you easily create
just a few inches below the surface. You are not strong enough to make the water
cavitate at 33', no one is. (I picked 33' because it one additional atmosphere.)
If you could find a swimming pool in a Himalayan village at 15,000' you would
find it extremely easy to create cavitation bubbles with your hand because it
would be only under 1/2 an atmosphere of pressure. We know from this thought
experiment that it's harder to make a fluid cavitate at higher pressure, and
it's easier to make them under lower pressure.
The next variable in the cavitation equation is the
pressure of the liquid. Water has a very low vapor pressure as compared to
most liquids including gasoline because of the polar nature of the water
molecules. Water is attracted to itself and resists cavitation. This is the same
property that accounts for surface tension and capillary action. Gasoline does
not have these properties and has a high vapor pressure meaning that it's easier
to create cavitation bubbles. Gasoline's vapor pressure increases when it get
hotter making it even easier to cavitate. Also gasoline vapor pressure can vary
quite a lot depending on the batch and still be in "spec".
We went over the importance of atmospheric pressure and how it's essential
for getting the liquid to the pump and how it's even more important when the
pump is pulling a suction lift. Then I gave a little dissertation on cavitation
and how it's affected by pressure and the property of the liquid called "vapor
As the plane climbs there is less and less atmospheric pressure available to
push the fuel up to the pump. The pump is still creating low pressure in the
suction line, but since there is less atmospheric pressure available the fuel
can not get to the pump as easily. At some point pressure drops below the fuel's
vapor pressure and cavitation bubbles begin to form inside the pump and in the
piping on the suction side. It's like moving you hand through the water in that
imaginary 15,000' Himalayan swimming pool. The same amount of energy that
wouldn't form cavitation bubbles at sea level will easily form them at altitude.
The formation of the bubbles is exacerbated by the fact that fuel has a
relatively high vapor pressure and will form cavitation bubbles easily. It's
further exacerbated if the fuel is hot which raises the vapor pressure further
The bubbles are formed in the pump and go into the pump from the suction
piping, but they immediately collapse on the discharge side because of the
higher pressure. What's happening is that the bubbles are occupying a volume
that would normally be occupied by fuel. The volume of the bubbles displaces the
fuel and results in a lower flow rate. Pretty simple actually.
The electric fuel pump is in a better position with regards to cavitation
because it sits lower in the plane and has a flooded suction. When you turn on
the electric fuel pump it restores some or all of the lost atmospheric pressure
and keeps the pressure in the mechanical fuel pump and suction lines high enough
to avoid the cavitation bubbles.
Now there is more going on here. For example, I don't understand why the
electric fuel pump doesn't always restore full fuel flow. According to some, the
electric pump has to be built so that it performs at the top of its spec or it
won't fully solve the problem. I'd think the electric pump would easily restore
the pressure and quell the cavitation. I also don't fully understand some of the
fluctuating flows and pressures that some pilots are reporting. It may be that
the cavitation bubbles are getting big enough to cause the pump to momentarily
As I have been thinking about it there are a couple more things are going on
that might be helpful if explained a little further. In the earlier sections, I
went over the fact that it's actually the atmospheric pressure that pushes the
fuel up to the pump. As you go up in altitude you have less atmospheric pressure
sort of holding the fuel together and at some point, the low pressure in the
suction line causes the fuel to form bubbles, that is what we call cavitation.
It would be accurate to say that the fuel is boiling, but I don't like to use
that term when explaining this because boiling implies heat. We live in a sea
level atmospheric world and we generally forget that a fluid will boil at any
temperature, the boiling point is established by the fluid's properties and
When fluid is moving through a pipe it creates turbulence. We all know,
because we are pilots, that turbulence creates drag. For the purposes of
understanding this, you can just think of it as drag, and the drag is further
lowering the pressure in the suction pipe and at the inlet to the pump.
Just like on an airplane any obstructions create further drag. In this
context the obstructions are things like elbows, valves, strainers, or fittings
that the fuel passes through. All these things are lowering the pressure in the
suction line and bringing the fuel closer to its vapor pressure with the
resultant bubbles and loss of flow.
As a pump expert, when I look at the suction pipe arrangement on a Bonanza,
it's a total mess. What you want is a short straight pipe with no obstructions.
What we have is a long pipe with a bunch of obstructions; the coarse fuel
strainer that is actually in the tank, the selector valve, the gascolator and
the screen, the electric fuel pump, and a whole assortment of elbows and
fittings thrown in for good measure. All these things are working against us and
adding to the cavitation problem.
Some obstructions are worse than others. Obstructions that are close to the
pump are much worse than obstructions that are far from the pump. For example a
90° elbow right at the inlet of the pump would be a disaster.
Why? you ask. Whenever the fluid passes an obstruction a low pressure area is
created just downstream. Think
Bernoulli. If the fluid is close to its vapor pressure the cavitation
bubbles will form at the low-pressure spot just past the obstruction. If this
low pressure spot is far from the pump, depending on all the factors surrounding
pressure and vapor pressure, the bubbles are likely to close back up before
reaching the pump. If the low pressure area is right in front of the pump, the
bubbles are likely to remain intact and enter the pump resulting in the loss of
flow for the reasons we've already discussed.
Obviously, our Bonanzas are certified aircraft and you just can't start
changing the suction piping around for kicks. Most of the problems inherent in
the suction setup are baked in the cake. However, there is some variability in
the hoses and fittings between the firewall and the pump.
Here are a couple of areas to examine:
Make sure the hose that leads from the firewall to the pump is in perfect
condition. If it's a Teflon hose it's probably Ok and you can easily inspect it.
If it's an old hose lurking under a fire sleeve it may be slightly crushed or
degraded. This is the most important part of the fuel suction setup since it's
close to the pump and needs to be tip-top.
Try to keep the fuel hose away from anything hot. Heat lowers the vapor
pressure of the fuel and takes it that much closer to cavitation. Even if I had
a Teflon hose, I'd consider putting a fire sleeve over it to help insulate it.
Also, check and make sure the cooling air duct for the pump is in place and in
good shape. The object of the duct is to help cool the pump, which fights off
Try to eliminate all the fittings between the firewall penetration and the
fuel pump. Try to have just one hose from the firewall penetration to the pump
with no fittings. Have the hose make long radius bends. If there is a 45° or God
forbid a 90° elbow actually attached to the pump
find a way to
get rid of it. You want the gas to flow into the pump with as long a straight
run as you can manage. I don't know what's possible here, however, my Bonanza is
NA so I'm not trying to pump 32 GPH to the engine at 15,000'. My pump is
probably cavitating when I'm at high altitude, but I can still get 12 -13 GPH to
the engine so I'm not worried about it. As a result, I haven't really looked at
the suction piping. But the TN folks may need to pay special attention.
All of the problems with cavitation are on the suction side of the pump and
in the pump itself. Once the fluid is through the pump and the pump is "pushing"
it, cavitation is no longer a factor. Newtonian liquids are incompressible so
you can push on them as hard as you want, you just can't pull them.
The mechanical fuel pump used in our Continental aircraft engines is a Gear
Pump and it falls in the family of pumps called Positive Displacement pumps.
Positive displacement pumps all share a common trait, they create a cavity then
crush the cavity.
This gives all positive displacement pumps two common qualities. First, their
flow rate is set by the RPM. The volume of the cavities is built into the pump
meaning that it will pump a certain amount per revolution. So, in order to
change the flow rate, you must change the RPM. Second, there is no theoretical
limit to how much pressure they can generate. Since liquids are incompressible,
the pressure generated when you crush the cavity is only limited by the strength
of the materials, and horsepower available. That's why there is always some
pressure relief mechanism on a Positive Displacement pump. And, that why we have
a fuel return line.
For any everyday example of a Positive Displacement pump (PD) think of a
pressure washer. When you let go of the trigger, the pump is still pumping. The
flow is just going through the pressure relief valve and is piped back to the
suction. If there were no pressure relief valve the pump would continue to build
pressure until something gave; the engine might stall because it just couldn't
make any more power, or something might break or slip like a shaft or a belt.
The pressure would try to go to infinity and it would only stop when if finds
the weakest part of the machine.
Our pumps are always pumping the same amount of fuel at a given rpm (assuming
no cavitation). I don't know what it is, but let's just say it's 40 GPH at 2500
RPM. So, if you are flying along at 2500 RPM burning 13 GPH, that means that 27
GPH is going back to the tank. Push the throttle or mixture in so now you're
burning 25 GPH, now 15 GPH is going back to the tank via the return line. The
total amount of pressure is set by the pressure relief valve, not the pump. The
pump could make lots more pressure as long as the fuel is getting to the pump.
And there we are, back to cavitation. The problem is that the fuel isn't
getting to the pump, it's boiling and creating bubbles. The bubbles in the pump
are occupying space that would be occupied by fuel if it weren't cavitating.
Result, the flow drops off.
This leads me to my last point. I consider myself to be somewhat of a pump
expert, but I'm not a Continental fuel injection expert. I have a good working
knowledge of how the system works, but I find myself occasionally puzzled.
The most recent example of my puzzlement is in a recent post that says "My
A&P has also talked to TAT and I understand that there is a larger orifice that
can be installed to help with this, somewhere in the engine." I can't think of
any reason an orifice would need to be installed anywhere in the system. If the
orifice is on the suction side of the pump, then it's in the perfect position to
create lots of cavitation bubbles in exactly the wrong place. If anyone can
explain what this orifice does I'd be interested in hearing about it
There you have it, my musings as a "pump expert" regarding cavitation, which
we pilots have always referred to as "Vapor Lock". I hope this narrative has
helped improve your understanding of our fuel system and the causes of
Just to add a bit more content to this narrative, I thought I'd post this
video showing FF fluctuations in a NA Bonanza at altitude with a high OAT.
I shot this video last summer at 12,500' with an OAT of 75 degrees. While 75
degrees may not sound hot, it's unusually hot for that altitude. Boost pump was
So what's going on here, why is it doing that? If you read the previous parts
of this narrative you've probably figured out that its cavitation, but why
It's actually fairly easy to predict cavitation, pumps generally have a curve
that shows the pressure where the pump will start to cavitate. You have to make
corrections depending on the vapor pressure of the liquid, but it's all
straightforward engineering work. What's hard to predict is exactly what it will
"look like". Will it be a straight drop off or will it pulsate? That depends on
the exact piping and pressures.
Say you are working with a bigger water pump that's electric driven,
something where you can actually hear the water flowing through the pump. It
will sound like you are pumping gravel.
As an experiment, say you want to force the pump to cavitate so you can see
what happens. You can do this easily by simply closing the suction valve
partway. Pretty quickly you will start to hear the gravel sound. As you continue
to close it, you will find places where the discharge pressure fluctuates wildly
and the needle is bouncing all over the place. What's happening is that vortices
are setting up in the pump. The vortices act like a little temporary plug that
drops the pressure even further. When the flow rate/pressure drops it changes
the conditions that set up the vortices and it breaks up. Then the conditions
are back to where they were and the vortices set up again. So it's the vortices
setting up and dissipating that's causing the fluctuation. The fluctuations can
be from several times a second to several seconds or even longer.
When you see a fluctuating discharge pressure gauge (that's what the Beech
factory gauge is after all), you have an excellent indication of cavitation or a
problem on the suction side of the pump. So we suspect cavitation because of the
temp and the altitude, the bouncing FF confirms it. Also, turn the boost pump on
and re-lean and it's gone, case closed.
Now the real question is "is it hurting anything"? After pondering this for a
while, I don't think so. As I pointed out in an earlier post, the energy level
in our little 1/2 gallon a minute pumps is quite low. I don't think the
cavitation bubbles that can be so destructive in larger high HP pumps are
actually damaging anything in our little pumps. I'm sure the spray pattern at
the injectors is oscillating, but apparently, it's averaging out because the
engine is smooth.
Of course, you can stop the bouncing needle right now by turning on the boost
pump, but that has its own risk.
On most NA planes you just have a single speed
pump and the single speed is high. If you turn the pump on and re-lean at
12-13,000' you must remember to turn it back OFF when you descend. Otherwise, if
you go full rich when you perform GUMPS or initiate a Go-Around, it may be so
rich that the engine quits. This could be a very bad if you go full rich for
landing and it quits while you are at a low altitude.
I've decided to just let it bounce unless it actually starts to affect the
engine, which I have never seen.
The TN guys have another set of issues as they try to keep 35-36 GPH flow
rate all the way to 18,000'. They must use the boost pump on low then high to
keep the cavitation at bay. Even with the boost pump on high they often can't
keep the high fuel flow at altitude.
Big thanks to
Jack for contributing his Vapor Lock/Cavitation narrative to CSOBeech!