P.A.S.T. Myth? Fact? Both?

[username]

Recent chance encounters with YT videos regarding critical engine (multi-engine training) introduced me to the acronym PAST (P-factor, Accelerated slipstream, Spiraling slipstream, and Torque). The use of PAST to explain certain factors related to the concept of critical engine seems to have become ubiquitous, but some of the explanations given in videos and other online presentations appear to be rather dubious. Since PAST wasn't used during my ME training (and later, instructing), I wonder where PAST originated. Perhaps someone on this board can direct me to the source.

-bob

 

[denverpilot]

No idea where it originated but the items in it are accurate. It’s a memory jogger, you’re supposed to know what those items mean.

That there’s YouTube videos that screw them up, really should come as no big surprise. LOL.

 

[denverpilot]

P.S. It’s as useful or dumb as TOMATOFLAMES if that helps keep it in perspective. Or any of the other memory joggers.

TOMATOFLAMES will fail you a checkride in a heartbeat if you don’t know what an MEL is, for example.

Memory joggers are just that. No more. You’d better know more than that.

 

[jsstevens]

[snip]
That there’s YouTube videos that screw them up, really should come as no big surprise. LOL.

I've done auto mechanics and construction most of my life (not professionally in many years) and I'm amazed at the stuff on YouTube. But I always start a video wondering if this is going to be useful or just entertaining.

 

[denverpilot]

I've done auto mechanics and construction most of my life (not professionally in many years) and I'm amazed at the stuff on YouTube. But I always start a video wondering if this is going to be useful or just entertaining.

Yeah I love YouTube for DIY but ... you do have to be careful.

I found easily ten videos about how to get the starter out of my Dodge and only ONE showed the correct way to do it completely from below.

Of course after I saw it I slapped my head as I reached for a longer ratchet extension and felt my way above the thing from the “wrong” side.

That ONE dude saved me a lot of cussing and pain. Well, AFTER I had already smashed “the boys” once on the fan shroud anyway. Hahahah.

Airplane videos, there’s good and bad... and about 99.9999% of them, even the good, wouldn’t know what a sterile cockpit was if one snuck up on them and bit them in the azz.

 

[flyingron]

It's not as dumb or contrived as ARROW, which I never cared for. There was never an explicit reg for W (other than it is included in O) and the second R has largely been obsoleted for domestic flying.

 

[FastEddieB]

I don’t think I’ve ever heard of “accelerated slipstream”. What is that?

My fourth was gyroscopic precession.

 

[James331]

Ahh hail rote knowledge!


If you understand the concepts you don't have to memorize all that junk.

And per tomatoflames, never taught it, for that one it's like NTSB 830 or some of the other stuff, it's something you need to be able to reference, not know in the cockpit. If something is inop, you look it up (just like the big boys do with MELs).

 

[denverpilot]

It's not as dumb or contrived as ARROW, which I never cared for. There was never an explicit reg for W (other than it is included in O) and the second R has largely been obsoleted for domestic flying.

All the cool kids around here at the clubs have shortened it to AROW. LOL.

I don’t think I’ve ever heard of “accelerated slipstream”. What is that?

My fourth was gyroscopic precession.

Air moving faster over the wing behind and around the operable engine on a prop plane compared to the other side.

Your aerodynamic mileage may vary, depending on nacelle design, wing design, and size of prop.

Doesn’t apply... much... to most jets. But then again, neither does the P one.

 

[Salty]

I don’t think I’ve ever heard of “accelerated slipstream”. What is that?

My fourth was gyroscopic precession.

I guess this is a multi-engine specific thang

 

[FastEddieB]

I guess this is a multi-engine specific thang

Yep.

I got my ME and MEI decades ago and may have simply forgotten the concept.

And thanks to denverpilot for being the first to explain it for me.

 

[jaybee]

Ahh hail rote knowledge!


If you understand the concepts you don't have to memorize all that junk.

And per tomatoflames, never taught it, for that one it's like NTSB 830 or some of the other stuff, it's something you need to be able to reference, not know in the cockpit. If something is inop, you look it up (just like the big boys do with MELs).

Heh, I was doing a 141 stage exam, instructor asked me what were required items were, I started naming some things from tomatoflames while simultaneously flipping to the appropriate section of the FAR. With a loud harrumph he exclaimed I'll never get a commercial license if I don't memorize tomatoflames. I reached for my copy of the PTS and asked him to please show me where it says I'm required to memorize anything. Another nose snort and an awkward pause later we moved on.

and here I sit not only comm heli & sel but also atp... (shoot I even opened up the books during my atp oral... unless it was a bold item that is...)

 

[denverpilot]

Heh, I was doing a 141 stage exam, instructor asked me what were required items were, I started naming some things from tomatoflames while simultaneously flipping to the appropriate section of the FAR. With a loud harrumph he exclaimed I'll never get a commercial license if I don't memorize tomatoflames. I reached for my copy of the PTS and asked him to please show me where it says I'm required to memorize anything. Another nose snort and an awkward pause later we moved on.

and here I sit not only comm heli & sel but also atp... (shoot I even opened up the books during my atp oral... unless it was a bold item that is...)

LOL. That’s funny. One of the DPEs around here hates the memory joggers. He likes asking people all the exceptions to the memory joggers when they use one.

 

[jaybee]

Yep.

I got my ME and MEI decades ago and may have simply forgotten the concept.

And thanks to denverpilot for being the first to explain it for me.

I'm a visual learner, this attachment may help

 

[jaybee]

LOL. That’s funny. One of the DPEs around here hates the memory joggers. He likes asking people all the exceptions to the memory joggers when they use one.

Yea, which is exactly why I opened the book because like the seatbelt required item has a date associated with it if my foggy memory is worth a dang, way too much bull to remember.

I concentrated on memorizing stuff like - what will allow me to continue an approach at MDA. Silly me I still don't know what tomatoflames stands for

 

[denverpilot]

I'm a visual learner, this attachment may help

I think that makes it worse. That’s just normal lift across the whole wing in that diagram with both engines turning. They didn’t show what it looks like with an engine shut down.

As Charles Barkley would say, Turrible. Just turrible.

 

[jaybee]

I think that makes it worse. That’s just normal lift across the whole wing in that diagram with both engines turning. They didn’t show what it looks like with an engine shut down.

As Charles Barkley would say, Turrible. Just turrible.

Better ?

LOL

 

[JAWS]

Yea, which is exactly why I opened the book because like the seatbelt required item has a date associated with it if my foggy memory is worth a dang, way too much bull to remember.

I concentrated on memorizing stuff like - what will allow me to continue an approach at MDA. Silly me I still don't know what tomatoflames stands for

Seatbelt required item date? I am not familiar with that one off the top of my head.

 

[jaybee]

Seatbelt required item date? I am not familiar with that one off the top of my head.

I had to look it up myself, foggy memory and all but yea - 91.205 (b)(14)

Tomatoflames also doesn't cover anti-collision lights & pyrotechnic devices(if needed) so not only do some archaic instructors insist on a rote memorization they are also forcing an incomplete answer...

 

[JAWS]

Gotcha!

 

[SoCal RV Flyer]

Better ?

LOL

Nice. As always, a picture is worth a thousand words...and better written!

 

[Tarheelpilot]

Yea, which is exactly why I opened the book because like the seatbelt required item has a date associated with it if my foggy memory is worth a dang, way too much bull to remember.

I concentrated on memorizing stuff like - what will allow me to continue an approach at MDA. Silly me I still don't know what tomatoflames stands for

I never learned it either. You are not the only one.

 

[denverpilot]

Nice. As always, a picture is worth a thousand words...and better written!

Except that the picture, even the edited one, still shows the same amount of lift being generated on both sides... LOL.

Your brain infers the idea better though, from seeing the “full thrust” drawing and the words from the memory jogger. It lowers the “mountain” of lift on the dead engine side in your imagination.

But the picture is still wrong. Hehehe.

 

[FastEddieB]

Except that the picture, even the edited one, still shows the same amount of lift being generated on both sides... LOL.

Is not the point that the lift is acting at a longer arm?

 

[denverpilot]

Is not the point that the lift is acting at a longer arm?

This is the “A”, accelerated slipstream that I’m talking about. Not the “S”, spiral slipstream.

The lift exists behind and around the operable engine and none on the non-operating side.

I don’t think the drawing is going for the “A”. It has some elements that hint at it, but it also has elements of “S” in the drawing.

(This is from memory of the drawing at the moment since I’m out and about on my phone. I can look again later when home.)

 

[jaybee]

I wouldn't say the drawing is wrong its simply showing two engines operating.

If we are going to parse things though, I never liked the term critical engine. I would say "more" critical engine

 

[username]

Post: “Why is the sky blue?”
Reply: “No idea, but here’s my opinion about boiled turnips.”
Gotta love internet forums.

I had turned to POA in hopes of learning about the origin of the acronym/mnemonic PAST as it pertains to multi-engine training. It seems that no one here knows either. It may be the most overwrought, overthought, and over-taught topic in GA training today.

With regard to the subject of critical engine and the way in which it’s often being taught, I offer the following:

1). Critical engine is applicable to aircraft certification only and has no real-world operational significance to pilots. None nil nada zed zip.

2). Of the multi-engine airplanes that I’ve flown, none of the manufacturers identified which engine (if any) they deemed to be critical, and why, in the AFM or POH. Therefore any discussion of PAST factors, at least with regard to those airplanes, is hypothesis and speculation.

3). Asymmetrical lift due to accelerated slipstream effects varies with AOA and may or may not significantly affect controllability at speeds approaching Vmc.

4). Except for airplanes such as the Beech 18 (for obvious reasons), spiraling slipstream plays no part whatsoever as it does not affect airflow across the vertical stabilizer in flight.

For the pilot, it oughtta be quite sufficient to understand the FAA definition of critical engine and how it pertains to published Vmc. During the oral for my MEI ride, the examiner, a very experienced and highly-respected industry veteran, summed-up the topic thusly [paraphrased]: the discussion is merely academic—if your airplane has two engines and one of them quits, the critical engine is the one that’s still running. Make sure you keep it running.

-bob

 

[denverpilot]

4). Except for airplanes such as the Beech 18 (for obvious reasons), spiraling slipstream plays no part whatsoever as it does not affect airflow across the vertical stabilizer in flight.

A nitpick. Every book and diagram that talks about spiraling slipstream talks about it striking the vertical stabilizer and rudder. I believe even FAA pubs show this. The diagram offered for the ATR also shows this.

You’re saying it doesn’t hit it at all unless the rudder is out further from the fuselage and all of those diagrams are wrong?

(I’m not saying I know, I’m saying there’s an awful lot of folks who say it does and you’re very experienced but the first person I’ve ever heard saying it doesn’t.)

Most of those diagrams show it exerting a force on the rudder and vertical stab, not really anything related to airflow over them. Pushing from the side due to the rotation of the vortex behind the operable engine.

 

[username]

A nitpick.

No.

Every book and diagram that talks about spiraling slipstream talks about it striking the vertical stabilizer and rudder. I believe even FAA pubs show this. The diagram offered for the ATR also shows this.

No, not with regard to this topic; no; and...nope.

You’re saying it doesn’t hit it at all unless the rudder is out further from the fuselage and all of those diagrams are wrong?

Anyone can draw a diagram, Nate. Think about it.

 

[FlyingTiger]

Post: “Why is the sky blue?”
Reply: “No idea, but here’s my opinion about boiled turnips.”
Gotta love internet forums.

I had turned to POA in hopes of learning about the origin of the acronym/mnemonic PAST as it pertains to multi-engine training. It seems that no one here knows either. It may be the most overwrought, overthought, and over-taught topic in GA training today.

With regard to the subject of critical engine and the way in which it’s often being taught, I offer the following:

1). Critical engine is applicable to aircraft certification only and has no real-world operational significance to pilots. None nil nada zed zip.

2). Of the multi-engine airplanes that I’ve flown, none of the manufacturers identified which engine (if any) they deemed to be critical, and why, in the AFM or POH. Therefore any discussion of PAST factors, at least with regard to those airplanes, is hypothesis and speculation.

3). Asymmetrical lift due to accelerated slipstream effects varies with AOA and may or may not significantly affect controllability at speeds approaching Vmc.

4). Except for airplanes such as the Beech 18 (for obvious reasons), spiraling slipstream plays no part whatsoever as it does not affect airflow across the vertical stabilizer in flight.

For the pilot, it oughtta be quite sufficient to understand the FAA definition of critical engine and how it pertains to published Vmc. During the oral for my MEI ride, the examiner, a very experienced and highly-respected industry veteran, summed-up the topic thusly [paraphrased]: the discussion is merely academic—if your airplane has two engines and one of them quits, the critical engine is the one that’s still running. Make sure you keep it running.

-bob

That's sounds like some pretty poor advise, hard to believe it came from an examiner. You can Vmc roll with the other engine running just fine in a light twin, especially when you marginalize the aerodynamic principles involved like you seem to be advocating.

Here's video of a twin with an inoperative critical engine crashing and burning while the other engine was operating right up to impact. I wonder if the pilot was instructed in the rather dubious theory that as long as the other engine is running, he doesn't need to be concerned with such "academic" notions as aerodynamics and single/critical engine procedures?

 

[username]

That's sounds like some pretty poor advise

You seem to have profoundly misunderstood the examiner's comment, the post, and the topic.

... single/critical engine procedures?

Pardon my ignorance, but what, exactly, are "critical engine procedures?"

-bob

 

[denverpilot]

Anyone can draw a diagram, Nate. Think about it.

So you’re saying the FAA diagram is wrong, the ASA diagram in their book on the topic is wrong, and all the others?

I’m not arguing it, I’m just saying you’re up against a number of training manuals that discuss it, reviewed by a LOT of people. I’m asking why you think they’re all wrong.

I’d like to see why it doesn’t apply if nearly everyone is teaching it.

 

[denverpilot]

That's sounds like some pretty poor advise, hard to believe it came from an examiner. You can Vmc roll with the other engine running just fine in a light twin, especially when you marginalize the aerodynamic principles involved like you seem to be advocating.

Here's video of a twin with an inoperative critical engine crashing and burning while the other engine was operating right up to impact. I wonder if the pilot was instructed in the rather dubious theory that as long as the other engine is running, he doesn't need to be concerned with such "academic" notions as aerodynamics and single/critical engine procedures?

Can’t find it now, but someone did the calculations of ground movement, camera location, and the aircraft, and that crew was well below Vmc. The aircraft kept slowing in an attempt to gain altitude.

Critical engine or not, that crew was not going to maintain control unless they got the nose down when they started the turn. Adding bank angle raised stall speed, and the left wing went first due to lack of accelerated slipstream.

Or put another way, nothing changes in that video if the crew turns right. They were still headed for a left wing stall condition.

Sadly, it had climbed well enough to that point that if they’d have kept it level for a while, they’d have a lot more altitude to trade.

But critical engine or not, they needed to get the nose down before banking. They were too slow.

 

[username]

So you’re saying the FAA diagram is wrong, the ASA diagram in their book on the topic is wrong, and all the others?

I cannot seem to find any FAA-produced content which relates accelerated and/or spiraling slipstream to the matter of critical engine. Sorry, I don't have an ASA book on the topic so I cannot comment regarding their material.

I’m not arguing it, I’m just saying you’re up against a number of training manuals that discuss it, reviewed by a LOT of people. I’m asking why you think they’re all wrong.

That's why I was motivated to write the original post--to initiate a forensic inquiry into when and where these concepts were introduced, and whether there was any real science supporting them.

 

[denverpilot]

I cannot seem to find any FAA-produced content which relates accelerated and/or spiraling slipstream to the matter of critical engine. Sorry, I don't have an ASA book on the topic so I cannot comment regarding their material.


That's why I was motivated to write the original post--to initiate a forensic inquiry into when and where these concepts were introduced, and whether there was any real science supporting them.

Fair enough. I’ll try to dig up some references when I have a minute. Very busy week.

Reason I would be a bit surprised if it’s wrong is I’m up to two instructors who used PAST and two high time DPEs who’ve accepted it on two different checkrides. I’d like to think it’s based on something traceable to FAA documents.

But I won’t have any time to do the research until this weekend at the earliest.

Maybe someone here will dig it up before then.

 

[nauga]

This is the “A”, accelerated slipstream that I’m talking about. Not the “S”, spiral slipstream.

The lift exists behind and around the operable engine and none on the non-operating side.

I don’t think the drawing is going for the “A”. It has some elements that hint at it, but it also has elements of “S” in the drawing.

(This is from memory of the drawing at the moment since I’m out and about on my phone. I can look again later when home.)

If you accept that P-factor causes a measurable yawing moment due to the higher angle of attack of the downgoing blade then you should recognize that the airflow behind the downgoing blade will be accelerated more than behind the upgoing blade, as that's what causes the yawing moment. That will offset the rolling moment due to "accelerated slipstream" as shown in Jaybee's diagram. That also means that the right engine in his diagram will have a larger rolling moment component due to accelerated slipstream in the critical engine determination.

Significant? Probably not. Flat out wrong? Also probably not.

Nauga,
wishing you a warm vector summer

 

[denverpilot]

Significant? Probably not. Flat out wrong? Also probably not.

Yeah that’s how I always looked at it. Multiple things going on at the same time that are all conspiring together to result in one thing, loss of directional control at low speed.

Didn’t seem like anyone minded even the tiniest possible thing in the explanations of what’s happening, on the ground. Maybe spiraling slipstream is small but still a factor.

In the air, you just see that the nose won’t hold a heading and you’ve run out of rudder, and possibly a roll toward the dead engine, and all that will save your butt is less power on the operable engine and more airspeed to counteract all those things, which you’re only going to get after you pull that power back, by getting the nose down.

Doesn’t really matter which one on the list is nibbling on your bum about to take a big bite.

They’re all working together. Plus your leg is tired.

 

[username]

I’ll try to dig up some references when I have a minute.

No need, unless you can help find an answer to the question posed in the OP or find those FAA docs. As for DPEs and checkrides, below are knowledge areas defined by the PTS:

• Factors affecting VMC and how VMC differs from stall speed (VS).
• VMC (red line), VYSE (blue line), and VSSE (safe single-engine speed).
• Cause of loss of directional control at airspeeds below VMC.
• Proper procedures for maneuver entry and safe recovery.

Assuming that an applicant were to demonstrate a solid grasp of those topics, throwing in a little sideshow about spiraling slipstreams and whatnot shouldn't be disqualifying.

Doesn’t really matter which one on the list is nibbling on your bum about to take a big bite. They’re all working together. Plus your leg is tired.

Precisely, and consistent with my argument that the whole discussion of critical engine and PAST is moot from the perspective of real-world operations.

-bob

 

[username]

If you accept that P-factor causes a measurable yawing moment due to the higher angle of attack of the downgoing blade then you should recognize that the airflow behind the downgoing blade will be accelerated more than behind the upgoing blade, as that's what causes the yawing moment. That will offset the rolling moment due to "accelerated slipstream" as shown in Jaybee's diagram. That also means that the right engine in his diagram will have a larger rolling moment component due to accelerated slipstream in the critical engine determination.

It's not so much a matter of accepting that these phenomena exist but whether they affect the flight characteristics of a specific airplane as predicted by the PAST hypothesis.

-bob

 

[FlyingTiger]

You can't just dismiss PAST as a moot point because that is essentially saying that understanding the aerodynamics of single engine flight in a light twin is a moot point and then we are back to just making sure the other engine keeps running. I think if you take a step back and look at the big picture, you might accept that the whole point of PAST is to demonstrate to a new ME student that you have a lot of factors working against you when you lose the critical engine and that makes the plane more difficult to control. I consider the instruction of PAST a situational awareness item more then anything. If that pilot in the video had it drilled into his head that he was in a much more precarious position because his left engine was out instead of his right, maybe he would of been more diligent and not been so fixated on gaining altitude like Denver noted above.