# Why do we need to wait for 30 seconds till the oil pressure gets into the required value ??

#### NightWalker

##### New Member
Hi,

In a Cessna 152, we usually wait for about 30 seconds after start up until the oil pressure rises.
My questions are,
1. Why do we need to wait?
2. Why exactly 30 seconds?
3. What will happen if we stay longer?

#### Roger Roger

##### Paid to sleep, fly for fun
Bruh, you need to do some reading about how a reciprocating engine works if that puzzles you.

#### NightWalker

##### New Member
Any specific article that you would recommend?

#### inigo88

##### Composite-lover
Sorry but I'm going to have to answer your questions with my own line of Socratic questioning, because it's important you think about and understand this.

NightWalker said:
1. Why do we need to wait?

What does oil do in a piston reciprocating airplane engine? What is its purpose? What does having oil pressure mean?

As a follow up, where do you fly? Is it a warm or a cold climate? What happens to the viscosity of oil when it gets cold? Would you expect oil pressure to take longer or shorter to come up after start on a cold day vs a hot one? Why?

2. Why exactly 30 seconds?

Are you continuing to watch the oil pressure gauge for exactly 30 seconds, every time, even if oil pressure rises into the green sooner? If so, you or your instructor is misunderstanding the intent of this practice. You should care about your engine achieving a certain oil pressure within a certain amount of time (which should be clearly stated in the POH), but if you keep watching for the full 30 seconds no matter what, you're doing it wrong.

This "X psi of oil pressure by X seconds" rule was given to Cessna by Lycoming, the manufacturer of the O-235 engine in your C152.

3. What will happen if we stay longer?

See question number 1, why does your engine need oil pressure?

Here's a partial list of important things in an O-235 that should have oil on them:

- Crankshaft
- Crankshaft bearings
- Connecting Rods and their bearings
- Camshaft (on top of the engine on Lycomings, far away from the oil sump for some reason)
- Lifters / Tappets
- Pushrods
- Rocker arms and valve springs
- Gears connecting the crankshaft, camshaft and mags at the back of the engine

If you don't know what those things are, read the FAA Airplane Flying handbook and google them.

#### jynxyjoe

##### Queso King
Hi,

In a Cessna 152, we usually wait for about 30 seconds after start up until the oil pressure rises.
My questions are,
1. Why do we need to wait?
2. Why exactly 30 seconds?
3. What will happen if we stay longer?
Good exercise for you will be to look all those up.

If you don't completely understand the 30 second bit i can give you two reasons i can think of.

#### NightWalker

##### New Member
Sorry but I'm going to have to answer your questions with my own line of Socratic questioning, because it's important you think about and understand this.

What does oil do in a piston reciprocating airplane engine? What is its purpose? What does having oil pressure mean?

As a follow up, where do you fly? Is it a warm or a cold climate? What happens to the viscosity of oil when it gets cold? Would you expect oil pressure to take longer or shorter to come up after start on a cold day vs a hot one? Why?

Are you continuing to watch the oil pressure gauge for exactly 30 seconds, every time, even if oil pressure rises into the green sooner? If so, you or your instructor is misunderstanding the intent of this practice. You should care about your engine achieving a certain oil pressure within a certain amount of time (which should be clearly stated in the POH), but if you keep watching for the full 30 seconds no matter what, you're doing it wrong.

This "X psi of oil pressure by X seconds" rule was given to Cessna by Lycoming, the manufacturer of the O-235 engine in your C152.

See question number 1, why does your engine need oil pressure?

Here's a partial list of important things in an O-235 that should have oil on them:

- Crankshaft
- Crankshaft bearings
- Connecting Rods and their bearings
- Camshaft (on top of the engine on Lycomings, far away from the oil sump for some reason)
- Lifters / Tappets
- Pushrods
- Rocker arms and valve springs
- Gears connecting the crankshaft, camshaft and mags at the back of the engine

If you don't know what those things are, read the FAA Airplane Flying handbook and google them.

Thanks mate, so simply we wait 30 seconds or the given time limit to make sure the oil circulates in the engine

#### CFI A&P

##### Exploring the world one toilet at a time.
Hi,

In a Cessna 152, we usually wait for about 30 seconds after start up until the oil pressure rises.
My questions are,
1. Why do we need to wait?
2. Why exactly 30 seconds?
3. What will happen if we stay longer?

A) If you are not mechanically inclined, definitely research the questions that Inigo asked.

B) It sounds as if you are a student pilot. Why wouldn't you ask your instructor these questions? I mean, you are sitting next to them killing time (30 seconds or so). Unless you are in a colder climate and the aircraft hasn't been preheated, you might be waiting a little longer than half a minute.

#### jynxyjoe

##### Queso King
Thanks mate, so simply we wait 30 seconds or the given time limit to make sure the oil circulates in the engine
Well think if a line broke, once you pressurize your gonna bleed out that oil quick. 30 seconds (maybe a little more) that supply is dry. If the supply leaks at the supply (not pressurized) it will take longer to see but if you can keep an eye on that oil press for first 30 seconds you can easily tell if ur pressurizing and if pressurizing a leak you'll know quick too...also lots of blue smoke.

#### milleR

##### Well-Known Member
Thanks mate, so simply we wait 30 seconds or the given time limit to make sure the oil circulates in the engine

One of two things is happening if you don't have indicated oil pressure within that timeframe- either you don't have oil pressure or the pressure indicating system is busted.

In either case you're not going flying.

#### Screaming_Emu

##### Joe Conventional
Sorry but I'm going to have to answer your questions with my own line of Socratic questioning, because it's important you think about and understand this.

What does oil do in a piston reciprocating airplane engine? What is its purpose? What does having oil pressure mean?

As a follow up, where do you fly? Is it a warm or a cold climate? What happens to the viscosity of oil when it gets cold? Would you expect oil pressure to take longer or shorter to come up after start on a cold day vs a hot one? Why?

Are you continuing to watch the oil pressure gauge for exactly 30 seconds, every time, even if oil pressure rises into the green sooner? If so, you or your instructor is misunderstanding the intent of this practice. You should care about your engine achieving a certain oil pressure within a certain amount of time (which should be clearly stated in the POH), but if you keep watching for the full 30 seconds no matter what, you're doing it wrong.

This "X psi of oil pressure by X seconds" rule was given to Cessna by Lycoming, the manufacturer of the O-235 engine in your C152.

See question number 1, why does your engine need oil pressure?

Here's a partial list of important things in an O-235 that should have oil on them:

- Crankshaft
- Crankshaft bearings
- Connecting Rods and their bearings
- Camshaft (on top of the engine on Lycomings, far away from the oil sump for some reason)
- Lifters / Tappets
- Pushrods
- Rocker arms and valve springs
- Gears connecting the crankshaft, camshaft and mags at the back of the engine

If you don't know what those things are, read the FAA Airplane Flying handbook and google them.

This is how one internets. Not "OMG NOOB, how u not know this?"

Further cementing my opinion that @inigo88 is a gentleman and a scholar.

#### inigo88

##### Composite-lover
I'll be the contrarian here and suggest that if the engine instruments are part of your regular scan, there isn't much to be gained by sitting and staring intently at the oil pressure gauge for a full 30 seconds after start. There's definitely a time and a place for that (basically all turbine engines) but I just don't think starting the mighty Lycoming O-235 on a warm day makes the cut. After start and good oil pressure, take a mental snapshot of your oil pressure gauge every few seconds during your instrument scan and look for trends.

I went back and looked and here's what Cessna has to say in the 1979 C152 POH (Normal Procedures, page 4-11):
After starting, if the oil gauge does not begin to show pressure within 30 seconds in the summertime and about twice that long in very cold weather, stop the engine and investigate. Lack of oil pressure can cause serious engine damage. After starting, avoid the use of carburetor heat unless icing conditions prevail.

It goes on to note that there are more specific procedures for cold weather starting which are noted in another section.

Back to OP's question #2, I'd like to point out that nowhere in the quoted C152 POH normal procedure above does it say exactly 30 seconds. I fear like so many things in aviation, a CFI read the "within 30 seconds", paraphrased it to a student who held onto it forever due to the law of primacy, who then became a CFI and taught it to their students, and on and on...

The reason I listed off so many of those engine parts in question #3 was simply because seeing those physical parts in action and understanding how that engine fits together and which parts are bathed in oil and why will be way more intuitive than regurgitating a canned answer like "because the engine needs oil pressure to work".

I know a lot of pilots' understandings of engines boils down to a black box that makes thrust and noise when you feed it money, but for the engineering and gearhead nerds among us they are really damn cool. On a personal note, I'm extremely grateful I got the opportunity to apprentice as an aircraft mechanic after high school (and kick myself daily for not finishing the A&P rating) because it both allowed me to pay my way through my flying lessons and gave me a hands on understanding of aircraft structures, systems and powerplants that pushed me to go back to school and become an aerospace engineer.

To that end, I know most pilots don't care to dig into systems stuff beyond the basic requirements, but I challenge you to try and find the opportunity to wander into a maintenance hangar sometime and look at all the "stuff" on your airplane taken apart and how it all fits together. It may challenge preconceptions of things you thought you knew, and you'll almost certainly walk away with a better understanding of the machine you're manipulating.

Since I can't provide you that experience through a web forum, I've attached two videos - a beginner one and an advanced/nerdy one. They both feature a Lycoming IO-360, which is fuel injected and makes more horsepower but has the same number of cylinders and a very similar geometry to an O-235.

1. The beginner video is made by Embry-Riddle, and has a very well done computer animation of the engine operating, with cutaways of individual components (14:34):

2. The advanced/engineering nerd version is "Skywardtech: Rebuilding Small Aircraft Engines (IO-360)". Just a dude from the 80s teaching you how to overhaul an IO-360 from start to finish in 36 minutes, including the infamous silk thread gasket at 4:33. I love this kind of stuff, and although I don't expect you to feel the same I hope a couple of my nerd brethren out there get a kick out of it too.

#### inigo88

##### Composite-lover
This is how one internets. Not "OMG NOOB, how u not know this?"

Further cementing my opinion that @inigo88 is a gentleman and a scholar.

Takes one to know one! Seriously, that means a lot coming from you man.

I benefited a ton from people taking the time to "pay it forward" on JC when my young mind was hell bent on soaking up all things aviation. Now it's my turn to put in some effort.

#### killbilly

##### Vocals, Lyrics, Triangle, Washboard, Kittens
I'll be the contrarian here and suggest that if the engine instruments are part of your regular scan, there isn't much to be gained by sitting and staring intently at the oil pressure gauge for a full 30 seconds after start. There's definitely a time and a place for that (basically all turbine engines) but I just don't think starting the mighty Lycoming O-235 on a warm day makes the cut. After start and good oil pressure, take a mental snapshot of your oil pressure gauge every few seconds during your instrument scan and look for trends.

I went back and looked and here's what Cessna has to say in the 1979 C152 POH (Normal Procedures, page 4-11):

It goes on to note that there are more specific procedures for cold weather starting which are noted in another section.

Back to OP's question #2, I'd like to point out that nowhere in the quoted C152 POH normal procedure above does it say exactly 30 seconds. I fear like so many things in aviation, a CFI read the "within 30 seconds", paraphrased it to a student who held onto it forever due to the law of primacy, who then became a CFI and taught it to their students, and on and on...

The reason I listed off so many of those engine parts in question #3 was simply because seeing those physical parts in action and understanding how that engine fits together and which parts are bathed in oil and why will be way more intuitive than regurgitating a canned answer like "because the engine needs oil pressure to work".

I know a lot of pilots' understandings of engines boils down to a black box that makes thrust and noise when you feed it money, but for the engineering and gearhead nerds among us they are really damn cool. On a personal note, I'm extremely grateful I got the opportunity to apprentice as an aircraft mechanic after high school (and kick myself daily for not finishing the A&P rating) because it both allowed me to pay my way through my flying lessons and gave me a hands on understanding of aircraft structures, systems and powerplants that pushed me to go back to school and become an aerospace engineer.

To that end, I know most pilots don't care to dig into systems stuff beyond the basic requirements, but I challenge you to try and find the opportunity to wander into a maintenance hangar sometime and look at all the "stuff" on your airplane taken apart and how it all fits together. It may challenge preconceptions of things you thought you knew, and you'll almost certainly walk away with a better understanding of the machine you're manipulating.

Since I can't provide you that experience through a web forum, I've attached two videos - a beginner one and an advanced/nerdy one. They both feature a Lycoming IO-360, which is fuel injected and makes more horsepower but has the same number of cylinders and a very similar geometry to an O-235.

1. The beginner video is made by Embry-Riddle, and has a very well done computer animation of the engine operating, with cutaways of individual components (14:34):

2. The advanced/engineering nerd version is "Skywardtech: Rebuilding Small Aircraft Engines (IO-360)". Just a dude from the 80s teaching you how to overhaul an IO-360 from start to finish in 36 minutes, including the infamous silk thread gasket at 4:33. I love this kind of stuff, and although I don't expect you to feel the same I hope a couple of my nerd brethren out there get a kick out of it too.

Why, oh why do you do this to me when I should be focusing on Technical Subject Areas and FOI (which I'm so tired of I am near puking - an example of Overlearning, I'm sure)

Because I really want to watch these videos now.

But I won't, I'll close JC and go back to studying....

(yes, I am studying my arse off because I am terrified of failing the CFI-A check ride)

#### trafficinsight

##### Well-Known Member
Why, oh why do you do this to me when I should be focusing on Technical Subject Areas and FOI (which I'm so tired of I am near puking - an example of Overlearning, I'm sure)

Because I really want to watch these videos now.

But I won't, I'll close JC and go back to studying....

(yes, I am studying my arse off because I am terrified of failing the CFI-A check ride)
You'll never pass if you don't watch these videos!

How's that?

Sent from my Moto Z (2) using Tapatalk

#### Screaming_Emu

##### Joe Conventional
Why, oh why do you do this to me when I should be focusing on Technical Subject Areas and FOI (which I'm so tired of I am near puking - an example of Overlearning, I'm sure)

Because I really want to watch these videos now.

But I won't, I'll close JC and go back to studying....

(yes, I am studying my arse off because I am terrified of failing the CFI-A check ride)

I read that as Chik-Fil-A ride at first glance.

Man things have changed...

D

#### Deleted member 27505

##### Guest
Sorry but I'm going to have to answer your questions with my own line of Socratic questioning, because it's important you think about and understand this.

What does oil do in a piston reciprocating airplane engine? What is its purpose? What does having oil pressure mean?

As a follow up, where do you fly? Is it a warm or a cold climate? What happens to the viscosity of oil when it gets cold? Would you expect oil pressure to take longer or shorter to come up after start on a cold day vs a hot one? Why?

Are you continuing to watch the oil pressure gauge for exactly 30 seconds, every time, even if oil pressure rises into the green sooner? If so, you or your instructor is misunderstanding the intent of this practice. You should care about your engine achieving a certain oil pressure within a certain amount of time (which should be clearly stated in the POH), but if you keep watching for the full 30 seconds no matter what, you're doing it wrong.

This "X psi of oil pressure by X seconds" rule was given to Cessna by Lycoming, the manufacturer of the O-235 engine in your C152.

See question number 1, why does your engine need oil pressure?

Here's a partial list of important things in an O-235 that should have oil on them:

- Crankshaft
- Crankshaft bearings
- Connecting Rods and their bearings
- Camshaft (on top of the engine on Lycomings, far away from the oil sump for some reason)
- Lifters / Tappets
- Pushrods
- Rocker arms and valve springs
- Gears connecting the crankshaft, camshaft and mags at the back of the engine

If you don't know what those things are, read the FAA Airplane Flying handbook and google them.

After you've sussed out those questions, here's another. After taking a sometimes looooong time to rise into yellow and green in certain temperature conditions, why might you then see the oil pressure spike into the high red zone as you add T/O power?

Last edited by a moderator:

#### jynxyjoe

##### Queso King
After you've sussed out these questions, here's another. After taking a sometimes looooong time to rise into yellow and green in certain temperature conditions, why might you then see the oil pressure spike into the high red zone as you add T/O power?
Ice in the oil getting pulled up into the straw like a homemade smoothie!!

D

#### Deleted member 27505

##### Guest
I'll be the contrarian here and suggest that if the engine instruments are part of your regular scan, there isn't much to be gained by sitting and staring intently at the oil pressure gauge for a full 30 seconds after start. There's definitely a time and a place for that (basically all turbine engines) but I just don't think starting the mighty Lycoming O-235 on a warm day makes the cut. After start and good oil pressure, take a mental snapshot of your oil pressure gauge every few seconds during your instrument scan and look for trends.

I went back and looked and here's what Cessna has to say in the 1979 C152 POH (Normal Procedures, page 4-11):

It goes on to note that there are more specific procedures for cold weather starting which are noted in another section.

Back to OP's question #2, I'd like to point out that nowhere in the quoted C152 POH normal procedure above does it say exactly 30 seconds. I fear like so many things in aviation, a CFI read the "within 30 seconds", paraphrased it to a student who held onto it forever due to the law of primacy, who then became a CFI and taught it to their students, and on and on...

The reason I listed off so many of those engine parts in question #3 was simply because seeing those physical parts in action and understanding how that engine fits together and which parts are bathed in oil and why will be way more intuitive than regurgitating a canned answer like "because the engine needs oil pressure to work".

I know a lot of pilots' understandings of engines boils down to a black box that makes thrust and noise when you feed it money, but for the engineering and gearhead nerds among us they are really damn cool. On a personal note, I'm extremely grateful I got the opportunity to apprentice as an aircraft mechanic after high school (and kick myself daily for not finishing the A&P rating) because it both allowed me to pay my way through my flying lessons and gave me a hands on understanding of aircraft structures, systems and powerplants that pushed me to go back to school and become an aerospace engineer.

To that end, I know most pilots don't care to dig into systems stuff beyond the basic requirements, but I challenge you to try and find the opportunity to wander into a maintenance hangar sometime and look at all the "stuff" on your airplane taken apart and how it all fits together. It may challenge preconceptions of things you thought you knew, and you'll almost certainly walk away with a better understanding of the machine you're manipulating.

Since I can't provide you that experience through a web forum, I've attached two videos - a beginner one and an advanced/nerdy one. They both feature a Lycoming IO-360, which is fuel injected and makes more horsepower but has the same number of cylinders and a very similar geometry to an O-235.

1. The beginner video is made by Embry-Riddle, and has a very well done computer animation of the engine operating, with cutaways of individual components (14:34):

2. The advanced/engineering nerd version is "Skywardtech: Rebuilding Small Aircraft Engines (IO-360)". Just a dude from the 80s teaching you how to overhaul an IO-360 from start to finish in 36 minutes, including the infamous silk thread gasket at 4:33. I love this kind of stuff, and although I don't expect you to feel the same I hope a couple of my nerd brethren out there get a kick out of it too.

After watching video "03 Engine" you'll realize why you want to get into turbines as quickly as possible.