B737-300 Critical Engine

[Hachiouji-shi]

Is there a critical engine on any of the Boeing 737s?
Any examples of a jet aircraft that has one ?
Thanks.

[Pablo K]

Turbojet/turbofan powered airplane critical engines are any LH or RH outboard engines... Number 1 or 4 for a 747... Vmca-Vmcg computations are based on the loss of any such engines. The "critical engine" is strictly an aerodynamic consideration...
xxx
Also consider another thing. Power ON or OFF stall speeds are the SAME in a jet airplane, since no propeller moves air over the wings, thereby no change in stall speeds.
xxx
Hope this answers your question...
xxx
Happy contrails -

[John Farley]

Mmmm.........

Don’t wish to nitpick but the exhaust from jet engines has swirl which can lead to slightly different yawing moments between engines if the engine is fairly close to the fuselage as in some twin installations. Whether this small effect is noticable to the pilot will be type dependant.

As to the extra matter of stall speeds raised by Pablo K I must really beg to differ there. While there is no prop-wash to add to the V over the wings there can be a very significant vertical component of jet thrust reducing the effective weight of the aircraft. At high angles of attack and high engine RPM this component of thrust can be considerable - as you will see if you do the trig.

[Pablo K]

Dear John Farley -
xxx
Please advise Boeing, the Argentina CAA and my airline (AR) that I indeed teach incorrect facts, and that all the manuals I use, or issued to me contain same erroneous information...
xxx
I will post a copy of your note on the pilot's bulletin board, among other jokes...
Incidentally, I recommend you also read "Handling the big jets" by the late D.P. Davies, ARB Editions, London.
End of subject.
xxx
Happy contrails

[Fiske]

I have always been told that it is possible to have a critical engine on a 737.

For example if there is a cross wind during take off. Which engine would be the least detrimental to lose in terms of handling the aircraft?

If the wind was from the left, losing the no. 1 engine would not be as bad as losing the no. 2. The yawing moment produced by the no. 2 engine would be counteracted a little by the crosswind, not compounded by it which would be the case if no. 2 engine was lost.

What do you think, was my instructor correct I think he was!!!

[Pablo K]

Dear Fiske -
xxx
That is correct statement about crosswind effect on Vmcg...
xxx
A crosswind is considered in CAA (UK) certification, not with FAA (US) certification.
Thereby you might see more conservative minimum V1/Vmcg with British manuals for same aircraft.
xxx
But a crosswind is one thing, a critical engine is one other...
You instructor gave you pertinent info.
Just the vocabulary of "critical engine", I believe, should not be used.
xxx
Happy contrails

[Wizofoz]

Also consider another thing. Power ON or OFF stall speeds are the SAME in a jet airplane, since no propeller moves air over the wings, thereby no change in stall speeds.

Not true, as, for a start, that is not the reason for a lower power on stall speed in a prop powered aircraft.

The reason for BOTH types is that at a nose high attitude associated with a high AOA, the thrust vector is tilted, so some thrust now works in conjunction with lift. Critical AOA is now achieved at a slower speed, as the thrust is producing some of the lift.

If you are teaching that Vs is the same power on or off, then yes, your manual needs revising.

[Pablo K]

Sorry, but I dont fly a Harrier, just a 25 years old 747...
xxx
I will inform Mr. Boeing that all the manuals I read are wrong...
These were 707, 727 and 747... oh also Mr. Douglas for DC-8...
And all the other books I studied since 1960...
xxx
Please tell me where I find a power OFF / power ON stall speed for my 747-287s?
These manuals fail to tell me...
xxx
The only jets with different stall speeds are airplanes with boundary layer control (bleed air)...
Maybe your Airbus machines have that... my Boeings do not.
xxx
Happy contrails

[Wizofoz]

Pablo,

You fly a 25 Year Old 747 whos' engines are tilted towards the ground at high attitudes. I fly 737s between 1 and 20 years old which do the same.

All the Stick Shaker and Stall Speed graphs I have in my Boeing FCTM specify Idle Thrust, as stall speed is different with thrust applied. Idle thrust stall speed varies with weight. If vectored thrust (By the deck angle, not adjustable nozzels) is supporting some of the weight, it has the same effect as a lighter aeroplane= lower stall speed.

[John Farley]

Pablo K

Please do not get upset!

Many aspects of the certification of aircraft are based on stalling speeds and it makes sense (that is gives increased safety) to use the highest possible stalling speed for any given weight and flap configuration. This will be always be the one associated with idle power and with the CG at the forward limit.

Manuals would be unnecessarily big and complex if they included stall data for all CG positions and power settings. But that does not mean that thrust levels do not affect stall speed by carrying some of the aircraft weight, it just means we do not take such matters into account when calculating stall speeds for various purposes as such effects will always be on the ‘safe’ side.

Regards

JF

[Pablo K]

Guys -
xxx
I spend many hours teaching classroom.
I do not tech aircraft systems, except flight instruments, etc...
My subject is performance...
xxx
With new hires, we review what is what as speed definitions.
My FCTM, and my AOM is the only bible I use...
I passed my ATPL back in 1968... Was not addressed then...
All that theory is fine and dandy...
Vectoring thrust, etc...
When that stick shaker goes at some 15 degrees attitude, I just apply a procedure.
If it would even make 1 or 2 KIAS difference, would not matter.
xxx
I have a little plane (Piper L-21)...
Stall power ON is 34 KIAS, power OFF is 39 KIAS flaps down...
THAT is a definite difference. In a jet transport, it is ignored, if any.
xxx
The Vref (Vat) is 1.3 Vs... the only speed I know for landing.
In cruise, we have 1.35 or 1.4 buffet protection...
Who cares power ON or OFF... no value whatsoever.
xxx
Sorry guys, I teach since 1975 or so... and too old to change.



Happy contrails -

[Andrew41]

B737 Critical Engine

To all:

Would appreciate any insight from any who can help with this.

The "critical engine in a crosswind" scenario has been mentioned, and I have been trying to get a grasp of that concept. I have a textbook that agrees with the poster's premise, that it's better to lose the upwind engine. But I just fail to see how a wind from that side doesn't complicate things immensely. For example:

Crosswind from the left. This requires right rudder to keep the airplane tracking straight on the runway, because it wants to "weathervane" into the wind.

Now, fail the left (upwind) engine. This creates a strong yaw to the left, further increasing the need for right rudder. I'm thinking you could run out of rudder in this situation.

OK, now continuing with my logic (and please remember I ask you to point out the flaws with it): Let's say you have that same left crosswind, you're holding right rudder to keep tracking straight on the runway, and now you lose the__ right__engine. This causes a strong yaw to the right. Now in this case you're getting help (too much!) with the need to hold right rudder due to the wind. Or in other words, the wind is causing you to weathervane to the left, but the failed engine is causing you to yaw to the right. This to me is the best scenario, because the wind is actually acting __against___the failed engine (due to weathervaning).

What am I missing?

Thanks again for any thoughts.

[Wizofoz]

Pablo,

Well lets see,

These figures are WAY approximate, but here goes:-

My 737 produces 40 000lbs thrust.

If it stalls at an AOA of 16 degrees, and the thrust line is parrallell with the chord line, the vertical component of thrust will be :-

40 000 * sine 16= 11025 lbs= 5000KG (Damn! I swear it just came out that neat!!!)

Looking at my (Idle Thrust) stall graphs, a reduction in weight of 5 000 kg at 54.5 tonnes, flap30 gear down reduces stall speed from 119kts to 112kts. Again, neatly, the same magnitude as the difference in your lighty.

Sir, you came on to this forum,brought up this subject, insulted anyone who disagreed with you, and when finally had the facts pointed out to you, responded by saying "It doesn't matter, and I'm too old to change".

If you are to old to stop teaching things which are incorrect, you should consider a career change.

[Pablo K]

Hola Wizo -
xxx
Fine and dandy - you get your QED...
It is not in my FCTM, nor AOM...
xxx
You can therefore assume that Vs computations/demonstrations are based in the most detrimental configuration (or power setting) when testing for FAR/JAR 25 by these flight test engineers. It certainly does not appear in any test of theory, nor flight test requirement that I practice in simulators.
xxx
If I can land an old 747-200 at Vref 130, I know its stall speed is assumed to be 100 knots. Shall I tell my guys that they can reduce Vref by, say, 5 knots, because of this component if they carry full thrust on their engines.
xxx
Every limitation is tested in its worse (or more detrimental) configuration... i.e. Vmca tests are done with CG at aftmost limit position. Then I assume it is power off for Vs demonstration.
xxx
As to being too old to relearn new tricks, I am definitely. But I am not questioning the material (i.e. Boeing FCTM) which I use as reference. I am just a "manager pilot training" with an obscure airline of South America. Declare us as flying with incompetent and poorly trained pilots... Thanks!
xxx


Happy contrails

[Paul Wilson]

Pablo
I don't think anyone was suggesting teaching anything other than the way Mr. Boeing says it should be taught.

John Farley just pointed out that you had made an error, an error that is probably not significant in line flying, as any error will be on the "safe" side.

Comments such as

I will post a copy of your note on the pilot's bulletin board, among other jokes...
Incidentally, I recommend you also read "Handling the big jets" by the late D.P. Davies, ARB Editions, London.

are ill thought out

John Farley is incidentally a very well respected pilot, suggest you do a search on Google for "John Farley harrier" and have a look at the results

[Pablo K]

Dear Andrew41 -
xxx
Your thinking is absolutely correct. Crosswinds, etc...
Jet transports have a VmcG (also our minimum V1 speed), but, at least we deal very little with that VmcG limitation, this at least with a 747...
xxx
My charts are for JT9D-7Q engines, our highest VmcG is listed as 128 KIAS... Our typical V1 speeds are in the order of 150 KIAS, with VR speeds in upper 160, lower 170 KIAS... therefore there is quite a marging for controllability.
xxx
I see this in simulator often. Should you set a very low weight, and speed combination (at or near V1 and VmcG), the airplane is extremely difficult to control. An engine failure at high V1, with substantial VmcG margin, is quite easy to demonstrate satisfactorily on proficiency checks. A "V1 failure" and engine out takeoff is easy to perform at a typical weight of 377 tonnes (833,000 lbs) weight.
xxx
If aware of a severe crosswind, say 29 knots (?) and dealing with a V1 near VmcG, obvious... it enters in my concerns and the before takeoff briefing... these are obvious things for guys flying the line... just should be mentioned occasionally while in recurrent training...
xxx

[john_tullamarine]

Pablo K, I see that you are new to PPRuNe ... welcome, sir.

However, I would like to note that many of the people who post, especially here in the Tech Forum, ARE Industry acknowledged technical experts. Most post under pseudonyms for various reasons, others post under their own names. We have quite a few people who are very experienced experimental and certification engineering test pilots (such as John Farley - a simple search on Google will give you an indication of John's background which, by anyone's standards, is more than impressive), others are from academia (including more than a few with PhDs, post doctoral qualifications, honorary doctorates and, to my knowledge, at least one fulltime university professor who holds an engineering chair at a well respected university. In your own fields of interest, we have a number of very experienced airline operations engineers, ground and flight training personnel ... the list goes on and on.

While I don't expect that you will, necessarily, agree with their opinions (one of the strengths of PPRuNe is a bit of cut and thrust discussion) I do ask that you consider the appropriateness of showing professional courtesy and respect to men and women who are, at the very least, your professional equal.

I presume that the nature of some of your posts is a consequence of your new arrival on site and that your style of writing will relax a little over the next few weeks. It is always easier to extract oneself from an unfortunate position if one's path into that position has left one with an escape route ....

Please be assured .. we seek and value your input .. but we like to keep the tone of things appropriately professional.

regards,

John



In respect of wind and Vmcg, the effect can be very significant, typically varying from around 0.5 kt/kt to something in excess of 1 kt/kt, depending on Type and engine configuration.

The critical case is a crosswind takeoff with V1 at or near Vmcg-limited and a failure of the upwind engine.

The big gotcha is that the onset of Vmcg-related control problems typically is fairly rapid (considering failure speed) and apparent over a small speed spread (sometimes as little as 5 kt can be the difference between a raised eyebrow and very sweaty palms). As a result, if the real world Vmcg is much elevated due to crosswind, then there is a VERY real possibility (under low V1 takeoff schedules) that the scheduled V1 may be somewhat less than the elevated value and could see the aircraft leave the side of the runway with predictable results - unless the pilot is quick enough to recognise the problem and abandon the takeoff - even if this means an inevitable overrun on a short, ASDR-limited takeoff.

Be aware that the usual certification is based on nil wind although the old BCAR requirements looked at the 7kt crosswind case. This is fine, unless you have the failure with a 30 kt plus crosswind .....

As to what you do in the short runway or low weight case in a strong crosswind ? .. there are various ways to mitigate the risk and these ought to be part of operator protocols and training.


Critical engine usually relates to the handling case but, depending on the rule set you are working with, it need not be just this simple.


It is useful to keep in mind that the OEM manuals are the result of a struggle between a desire to provide information and the need to keep an eye on potential litigation ... ergo .. such manuals are not always as useful or detailed as they might be. In particular, the OEM must produce a manual which is suitable for all pilots regardless of resources, experience, and skill levels. In addition, if one seeks the physics behind the observation, then the OEM manuals are not the place to seek such explanations.

[quid]

Andrew 41-

You've got it correct. If you could choose which engine will fail, you'd always choose the upwind engine. You have lots more rudder to use to keep it rolling down the center. We have a saying in 3 engine ferry operations, "Good engine, good wind. Bad engine, bad wind." You don't want to tempt fate by taking off with an inoperative engine on the upwind side.

Pablo K-

You might try this exercise the next time you're in the sim. Set up for level flight at say, 10,000 feet in the landing configuration. Gradually let the airspeed decay to stall speed (not stick shaker)while gradually reducing to idle power. Note the speed. Now, do the same thing again, but once you get behind the power curve, gently increase the thrust to max power. You'll probably find that you can maintain level flight at an IAS 10 knots or more slower than power-off stall speed.

PS- usually when John Farley speaks, pilots listen.

[Hachiouji-shi]

thanks

John Tullamarine,

So, the critical engine on the B737 is ONLY applicable when the upwind engine fails with V1 at or near Vmcg ? What about in nil wind conditions, theoretically ?

Thanks!

[john_tullamarine]

No .. this is typical of the problems between the artificial world of certification and flight test and the real world of aeroplane driving ....

The value declared in the AFM for Vmcg will be appropriate for a set of conditions, including nil wind (US certification). But the story is a little more complex than that ...

Consider certification and flight testing, generally. If we were to run a test program or a certification exercise in the absence of defined "standards" or "assumptions", then we would need to pad out the work report(s) with a whole bunch of information so that the reader would know what the conditions, assumptions, and so on which went into the work were. Making comparisons of different tests etc. would be a nightmare workload. Such a system would last all of about 5 minutes ....

So, we refer performance data to (nominated) "standard" conditions (which represent some sort of average atmosphere) and use defined conditions for tests such as Vmc and Vmcg.

The problem arises in that the values so obtained relate only to the presumed and defined conditions. So, the AFM Vmc/Vmcg figures are reasonably conservative but not necessarily always conservative, when measured against normal operations.

It is quite "legal" to takeoff at Vmcg-limited minV1 in maximum crosswind, from a low elevation aerodrome, with maximum aft CG and rated thrust ... just don't lose the upwind engine approaching V1 and expect to be able to continue the takeoff without some handling problems and wide eyed looks of amazement ... I can well recall when I started to use this sort of exercise for familiarisation in endorsement training programs ... provided that the particular simulator has a half decent program for this area of the envelope it opened more than a few eyes to the potential for high pulse rates if a pilot doesn't keep his/her wits about him/her. On simulators which don't it often is a total waste of time.

It comes down to a need for the commander to have an appreciation of the limitations inherent in certification matters as they might affect line operations.

So far as "critical engine" is concerned, think in terms of which engine (having failed) is going to give you the greater problems ... this is the critical engine. However, for certification, the conditions under which this and any certification parameter is determined are constrained ... although there is nothing to stop an OEM from making more extensive investigations (except the fact that it might cost a lot of extra dollars).