View Full Version : Forward Slipping a 737-800

23rd Jul 2002, 18:12
Sometimes you just get absolutely "crap-vectors" from ATC. Sometimes you find yourself high and steep on final, and need to lose altitude. As new pilots, we're introduced to the Forward Slip manuever. It's a basic skill we seldom call upon flying large aircraft. Recently, I've begun to wonder about the applicability of this manuever in larger aircraft, with the recent Airbus crash in NY.

I've forward slipped the 737-800 a couple of times in the past. At final approach airspeeds it seems to handle quite nicely, and is very effective when "S" turns and "360's" aren't an option. I know of no specific prohibition on this maneuver from Boeing. But recently, I wonder if I might be putting and an undue twisting moment on the fuselage. Does anybody know what Boeing's position is on this manuever?

23rd Jul 2002, 19:28
I'd be more concerned with the passengers' positions than Boeing's...

How many passengers would be comfortable in a slip that was significant enough to increase the rate of descent moreso than full speedbrakes, idle thrust, and max flaps for the current speed?

23rd Jul 2002, 22:33
Granted, it feels a little odd when you intially enter a forward slip. It's about the same sensation as kicking out the crab angle on short final in a X-wind landing. There is a high rate of descent associated with a forward slip but that's the point of doing the manuever. You can look in just about any basic pilot training manual and see that its a taught manuever.

Anyways, I've flown some airplanes that strictly prohibit the forward slip manuever in certain flap configurations. Boeing makes no mention of the maneuver at all. I was hoping this site might have an aeroengineer or other specialty that would have more insight into this.

23rd Jul 2002, 22:48
Q: in a jet, does a slip lead to compressor stalls?

24th Jul 2002, 00:43
Id be very dubious about applying slip if the standard operating manual doesnt mention it.... Its a bloody good trick to get your sailplane on the deck ASAP, however, PAX generally get pi**ed off when they have to lean sideways to drink their G and T.
Reasons behind flap limitation to aircraft sideslip occur as a result of decrease in control surface authority (horiz. Tailplane mainly) and even stall of the rudder. Not Healthy. Buffet Generated by high alpha manoevers on flexible tailplanes can significantly decrease the fatigue lives of their constituent components, as per tornado GR1, B707 F18, F16 etc etc...
As regards buffet entering engine intakes, its probably studied in the design process, however, having a background investigating Resonance induced blade failures and its complexities, it would definately be worth asking the chaps at boeing.... I cant imagine them looking at it in too great detail, pilots are supposed to stick to straight and level as much as possible to keep the boffins happy.Could always try it in the sim, or just ask your fleet chief instructor...
From an engineering standpoint, its probably best to avoid, as you are entering flight envelopes that are cleared for flight, but not as standard procedure and thus can have no idea as to the long term effects.
Hope this was helpful

24th Jul 2002, 08:12
I don't think inducing high RODs close to the ground is very healthy. Keep to your SOPs as far as you can and I'm pretty sure that no sensible airline would include forward slips as an approved method to regain your approach profile. I'm sure the regulator would have something to say about it too...

24th Jul 2002, 08:49
If things are that desparate why not go-around?

Hew Jampton
24th Jul 2002, 14:30
When asked about this a couple of years ago, UK CAA test pilots said that slipping up to max rudder does form part of the certification flight testing. That doesn't necessarily mean that it's a 'good thing'.

24th Jul 2002, 17:27
I agree with Mr. Mouse.

A rudder is designed to cope with the stress of a deflection, but it is not designed to cope with that stress continiously.

24th Jul 2002, 17:49
I was taught thta it is absolutely not done to sideslip larger a/c. If s-turns and flaps 40 don't do the trick then you're in go-around area anyway.

24th Jul 2002, 17:50
I wouldn't try S-turns in a congested area.

Eli Vator
24th Jul 2002, 19:28
How strange that this topic should suddenly appear!!

I was sitting at the holding point at Orange H.Q. a couple of days ago waiting for an aircraft to land. (We presumed this because ATC had merely told us to hold position when we reported ready for departure).

Gazing up the final approach path we spotted an aircraft in the distance that appeared to be manoeuvring randomly (or so it seemed) and getting closer. We therefore surmised that we were being held because of an unidentified intruder.

As it got closer and much to our surprise, it became evident that it was one of Stelios’ finest, vigorously performing a series of “S” turns and side-slips in order to lose height. The determination to “get in” was obviously paramount and quite alarming.

Correct me if I’m wrong, but the only reason that I can think of to perform such prolonged and extreme manoeuvres would be in the “loss of all engines” case.

Unless there are pertinent details that I’m not party to, this was the worst case of commercial flying I’ve seen in more than two decades.

As M. Mouse quite rightly suggested, the PROFESSIONAL thing to do is GO AROUND.

24th Jul 2002, 20:08
Hmm....Seems this topic is skewing off from the intent of its original post. I'm interested in the opinions of aircraft structual engineers and the like. The fact is...the Forward Slip IS a valid flight manuever.

I'm not interested in the opinions of pilots who keep their bank limiter set to 10 degrees in the traffic pattern and who don't know the difference between a Forward Slip and a Side Slip. Who consider a standard rate turns, "S" turns and "360's" to be the realm of aerobatics. Who "**** their pants" with the loss of their FDs. You guys can "kiss my @$$". BTW, Eli Vator, that wasn't me.

So....If you have any knowledge from a structures or areodynamics standpoint, "like UAM's post". I'd like to hear from you. Otherwise, move on!

24th Jul 2002, 20:37
Given recent events (A300 tail failure), I would be dubious about doing such a manueovure. What happens if you are in a slip with right rudder, and the right engine stalls / fails ?

24th Jul 2002, 20:49
OMG! Your throttles are at idle when you Forward Slip! For the LOG, if you have no knowledge of aircraft structures, please don't waste my time.

You know what. See ya! ....I'll email my concerns to Boeing.

24th Jul 2002, 22:20
Excuse my ignorance I know what a 'side slip' is but what is a forward slip?

24th Jul 2002, 22:28

24th Jul 2002, 23:17
Dear DVT,

I'm not interested in the opinions of pilots who keep their bank limiter set to 10 degrees in the traffic pattern and who don't know the difference between a Forward Slip and a Side Slip. Who consider a standard rate turns, "S" turns and "360's" to be the realm of aerobatics. Who "**** their pants" with the loss of their FDs. You guys can "kiss my @$$". BTW, Eli Vator, that wasn't me.

My my such language... I don't consider S-turns aerobatic, but I do consider S-turns on final approach to cause an unstable approach, and I do consider forward slipping to cause the pax to be uncomfortable, so better to go around, or to ask for a longer vector to final, then to start steering around the localizer or to get your airplane down.

As I said, on the structural level you are asking for: A rudder is made to cope with the stress in an Engine-Out-situation and for decrabbing and the like, not to cope with continuous or repetitive stress in various approaches where your own true-airman-I-use-30-degrees-of-bank-descent planning wasn't right on the spot.

I do think the opinion of less brave pilots who stick to SOPs could be very valuable, and though sidewards slipping or forward slipping is possible in a 737 it might not be ideal or worse even it might cause structural damage when done too often.

Almost all posts in this thread include the question "is it wise to perform such a manouvre near the ground?" or "why not perform a go around and try again?"; before attacking all these professionals as non-airmen, or before reducing your attention to Wild E Coyote-pilots who don't give a damn about SOPs, you could consider the experience that is behind these questions.

On top of that: Shouting 'OMG' (o my god?) if somebody talks about an engine failure during such a slip manouvre: yes the throttles are idle if you are to lose altitude, still, especially in the approach, with the throttles at idle you have about 35% N1, which is lost during an Engine Failure, and even for that amount of thrustloss you need to correct with the rudder. Might seem to be a minor detail, but not less important.


PS By the way, I lost my flight director once and didn't "**** my pants", but landed uneventful.

25th Jul 2002, 09:58
To quote Joe Sutter, late of Boeing some years ago, when I this asked the question, the answer was: “if the manual doesn’t prohibit it, the can do it”.

It was prohibited on the 707, because at that time SFAR 422B Certification did nt include it, any wiping engines of a pylon in an unintentional slip was demonstrated on more than one occasion.

All the 747's, up to the 744 slip quite nicely, and the quite moderate actual angle of bank and out of balance forces than can produce quite a useful increase in RoD, if you get it right, won’t spill anybody’s G&T.

All the rules you were taught in basic training still apply, regardless of the size of the aircraft.

As for the suggestions that a missed approach is the “professional” answer, firstly I hope nobody is suggesting continuing this or any similar maneuver within the “stable approach” envelope, usually 1000 agl and down.

Secondly ATC planning only ever accommodates a missed approach a second best option, ATC can suffer from getinitis as badly as a pilot.

Tootle pip !!

25th Jul 2002, 12:14
I, like M. Mouse, had no idea what a "forward slip" is ... and, having looked at the page referred to by dvt, I still don't ..... could someone please explain to me what the difference in the two examples is ? I can see absolutely no difference between the two .... or am I just too dumb to see a subtlety in the detail ? At the risk of having to eat my words .. as I have to do from time to time ... is this set to become one of those wonderful myths of aviation ?

As a general practice, I would be very wary of adopting the attitude that, if the book doesn't prohibit it, then it is OK to do something .... the absence of prohibition in respect of a specific matter may only indicate that it didn't come up in the certification process .... far better, I suspect, to follow the guidance material without too much potential experimental flight testing in line operation ....

25th Jul 2002, 13:07
This may be akin to the alter-boy telling the priest how to conduct high mass

Forward Slip
In a no-wind situation, you’re flying along in your Spad and decide to land in some green pasture in Flanders. Noticing you’re a bit high, you stomp on the rudder bar while adding in an appropriate amount of opposite aileron to counter the yaw. The relative wind is now displaced from the centerline of the aircraft and a wing is lowered and forward of the other wing. This lowered wing is in the direction you’re traveling. With the wind displaced from the nose you have increased your wetted area, increasing your drag coefficient, and with the same power, increased your rate of descent – no flaps needed. The direction of travel is off-set from the nose of the airplane.

Side Slip
On a windy final approach you crab toward the crosswind, away from the runway. Moments before touchdown you may desire to align the longitudinal axis of your airplane with the centerline of the runway with minimal transverse movement. You apply a bit of rudder pressure while countering with opposite aileron. The longitudinal axis is aligned with the runway and thankfully is in the direction you’re traveling. You have succeeded in negating the effects of the cross-wind, but you’re not level, the up-wind gear should squeak on prior to the down-wind gear. The direction of travel is parallel with the nose of the airplane

Favorite Slip
When my girlfriend forgets to wear hers with a translucent skirt.

I have read various accounts of slipping heavy jets, B747 and DC8s, and understand they exhibit very docile qualities in the slip. It may be a fine maneuver for our freight brethren or when empty, but it’s not a normal maneuver for me with passengers on board, and will not do it when transporting such.

25th Jul 2002, 13:56
Is it not true that the Air Canada 767 that ran out of fuel in 1983 did a monster side-slip to get into Gimli?

Obviously with no engines he had only one chance to get it on - as a lowly PPL I'd guess that would be the only time you'd ever want to heave a 767 on it's side.

Even in a Tomahawk, the momentum caused by side-slipping can get a bit hairy of you're not, pardon the pun, 'on the ball'...

25th Jul 2002, 14:22

I far prefer your third explanation and concur without any reservation ... however, as to the first and second ...

Could we revisit the very nice graphic at the site to which dvt directed our attention ..... which had, if I recall correctly, two pictures ... that on the left described as a "forward slip" .. and that on the right as a "side slip".

Might we make some minor alterations to the "side slip" image and do an interesting thing .... ?

(a) erase the reference to wind. Does anyone take issue with my suggestion that an aeroplane, other than via sophisticated electronic wizardry, knows naught about a steady wind ? If so, then the reference to wind is irrelevant.

(b) having removed the irrelevant reference to the wind, draw in an arrow which might represent the path, or track, which the now wind-less aeroplane might, perchance, take ....

(c) now, with scissors keenly sharp, cut out the resulting picture, apply some magic spray to render it somewhat transparent, and

(d) move the said somewhat transparent image to the left and superimpose it on the "forward slip" image

(e) rotate the superimposed image such that the arrow which you drew .. now is aligned with the runway

To my simple engineer's mind ..... the two appear to be quite similar .......

... which brings me back to my original question .....

... what is the difference between the conventional side slip ... which is well understood ... and this newfangled "forward slip" ?

I think that it is all waffle ....... any takers for a discussion on the pros and cons of my position ?

Hew Jampton
25th Jul 2002, 16:52
I think that in some circles the term 'forward slip' is now applied to what used to be called 'side slip', ie the aircraft tracks a straight line over the ground, usually the approach centreline. In these circles, 'side slip' has become slipping laterally across the ground, starting to one side of the extended centreline but arriving over the runway on the centreline, assisted where possible by any crosswind.

From Handling the Big Jets , by Dai 'The Book' Davies, ARB Chief Test Pilot: "It is wrong in principle to allow a swept wing aeroplane to suffer significant angles of sideslip". Good enough for me, except, as mentioned earlier, when faced with a no-thrust approach.

25th Jul 2002, 17:04
I don't claim any real-world experience beyond low hours solo glider flying, but I'm with John 100% on this one.

25th Jul 2002, 18:46
"From Handling the Big Jets , by Dai 'The Book' Davies, ARB Chief Test Pilot: "It is wrong in principle to allow a swept wing aeroplane to suffer significant angles of sideslip". Good enough for me"

What do we PROFESSIONAL pilots call this principle..."The Because I Said So Principle." I want to hear it from the lips of a structures engineer, that "we didn't design the airframe to handle the stress of this manuever. I suspect this may be the case...at least over the long term.


Side Slip is a precision manuever to land when you have drift. You kill the drift with rudder and fly the appropriate amount of aileron to a landing. Here the wing gets dipped into the wind and you touch down on one wheel first.

Forward Slip is a nonprecision manuever when you need drag. It is an intentionally uncoordinated turn but you don't turn. You slowly feed in rudder and use aileron to level the wings. You keep your forward vector. The ailerons offset the rudder for zero turning effect. The vertical fin and fuselage are now exposed to the relative wind creating lots of drag. And I suspect, lots of stress.....more than Side Slipping anyways.

Which brings me to the reason for my post. Where am I in this stress envelope, when I forward slip at final appraoch speeds? This is the question which I submitted to Boeing. If they answer me, I'll post.

25th Jul 2002, 22:45

I appreciate that your question was about the structural implications but unless I was about to crash I cannot think of any occasion when I would choose to carry out such a manouevre.

For what it is worth I also happen to agree with JT, there appears to be no fundamental difference between so called 'forward slip' and 'side slip'.

25th Jul 2002, 23:01
Sideslips and forward slips are very similar, but the previous explanation is correct.

The generally accepted definition is that the side slip aligns the longitudinal axis of the aircraft with the runway and utilizes bank angle to maintain track on the centerline (crosswind situation). The actual path through the air is to the side to compensate for the wind. Zero wind and you're not heading for the runway, but going to the side of it.

A forward slip involves applying rudder in one direction and enough opposite aileron to prevent a turn. In zero wind you will still be tracking the centerline of the runway. Usually you apply full rudder and enough aileron to compensate.

These are the standard definitions used by FAA, etc.

I think the main difference is that in the sideslip you are using enough aileron to control the drift and just enough rudder to keep the longitudinal axis aligned with the runway. In a forward slip you are using a whole lot (to the stop in a light airplane) of rudder and then aileron as required to prevent the turn. Consequently, you generally use a whole lot more rudder in the forward slip. I would agree with those that say that if you need to do this to lose altitude you've botched the approach and should just go around.

25th Jul 2002, 23:21
With apologies for paraphrasing one of the the old wise Yank pilots on this forum some time ago who offered this gem on another not dissimilar subject..
"Boy, if the manufacturer don't recomend doin' something,dont go stikin' yer dick in there!! ":D

26th Jul 2002, 02:08
With a pointer to the FAA (thank you, Prof2MDA) as the source of this terminological excess, I did some research (using my trusty US-sourced CD containing lots of FAA bits and pieces). A bit of study is always a good thing as one invariably learns something in the process.

The term, "forward slip" indeed is to be found within some areas of the FAA flight training and flight standards bookwork.

For the interest of others who might, like me, not be working within the US environment and not have come across the term before, may I cite some references which you might care to examine -

(a) AC 61-89D, Appendix 1, Lesson #37

(b) FAA-H-8083-3, Chapter 7, from which comes the graphic referred to by dvt (at Fig 7.6). Interestingly, this text makes it clear that the two are the same, except for the (groundbased runway frame of reference) observed track.

(c) Order 8400.10, Volume 5

(d) FAA-S-8081-5C, -6A, -14

(e) Dictionary of Aeronautical Terms

If I may quote from the last document (even if I might take the view that the definition is somewhat inadequate) -

"slip (aircraft flight maneuver).

A maneuver in which the aircraft moves through the air sideways, rather than straight ahead. Slipping produces a large amount of drag so the airplane can descend at a steep angle without gaining excessive speed. An airplane is slipped by crossing its controls, using right rudder and left aileron.

A forward slip is one in which the aircraft turns its side into the wind, but continues in forward flight.

A slide slip is one in which the aircraft continues to point straight ahead, but slips to the side."

Do I have to eat of the weekly umbles pie as befits my lowly status for not knowing more about this matter ? .... I guess so.

Do I still think that the two are the same thing waffling as distinct entities ? .... absolutely.

Is either view more important than the other in the overall scheme of things ? .... probably not.

So far as structural implications might be concerned, I would be less concerned with moderate slip angles than with rapid and varying rudder inputs. There is, also, the possibility of asymmetric stall and unintended spin if the manoeuvre is done at lowish speed and in a ham-fisted sort of way. As always, we in the field do not have access to the manufacturer's design records ... therefore, if something is not prescribed in the manufacturer's documentation then the manufacturer ought to be consulted prior to embarking on one's own little flight test card.

In sim training, I have seen some prodigiously extreme slip angles employed successfully for the purpose of recovering the aircraft from an all-engines failed scenario.

In real life line operations, though, the intentional use of slip (other than that which necessarily is used during the final stages of a crosswind approach and landing) is both unpleasant and unnerving for the SLF and probably has no routine place in large aircraft operations.

Having said all that ... as a very young and comparatively inexperienced pilot many years ago on glider tug operations using the lovely SuperCub .... nothing since gives me the routine delight as that which I derived from an extreme sideslip down to the flare with the tow rope just clearing the boundary fence by inches .... and occasionally leaving the last few inches on the said boundary fence. Probably not very sensible flying and I don't think that I would have any interest in doing so any more .. but it was all rather good fun at the time ...

26th Jul 2002, 03:08

If the manual doesn't specifically prohibit it, then it's OK to do it? I'm finding it personally difficult to buy into that line of reasoning. The manual would have to be many thousands of pages thick, and even then, I think something would be missed. Much safer (from the manufacturer's legal liability standpoint) would be to say what you "can" do, and anything else would be on your head.

Aviation Week, in the aftermath of AA587, has done a number of excellent, eye opening articles on rudder usage. Some of which bring out statements from the structural design engineers saying words to the effect of, "..we never expected the pilots would do that...".


If Davies says it's not a good idea, that's good enough for me (and untold thousands of other professional pilots). If you can't learn from him (and others like him), who are you going to learn from?

For what it's worth, my company prohibits slips (to lose altitude) across our fleets. I was part of that decision, and I'm comfortable with it.

26th Jul 2002, 03:43
"For what it's worth, my company prohibits slips (to lose altitude) across our fleets. I was part of that decision, and I'm comfortable with it."

And this decision was based on what?.........The "Because I said so principle." I'm curious...what's your company's policy on hand flying, raw data approaches, appropriate levels of automation, and the like? I've seen many once fine aviators who've forgotten the BASICS of flying. Can you believe a commerical airline pilot with over 25,000hrs of instrument flying can have NO functional crosscheck. They can't trim hands off in a turn or for level flight. They can't hand fly and think, they're task saturated. Guess what? Their out there. And I find they work for companies that set policies like yours. I'd be careful of setting company policies that stiffle pilot skills.

Every pilot needs a bag of tricks. And he needs to inventory that bag from time to time. They don't need micro-managing desk jockeys who take stuff out of their pilot's bag of tricks. BTW, some of the worst flying pilots I've ever seen have been Check Airmen/Instructors. I know what's in my bag? If you're gonna take something out of my bag, you better have a damn good reason. Some better than "Because I said so".

26th Jul 2002, 04:37
"All things and all people in life have to sink or swim on their own merits, not their reputation; that just as a wise man can say a foolish thing, a fool can say something wise." Vincent Bugliosi

26th Jul 2002, 04:49
I believe the dicision was based on sound reasoning, with safety being primary. Believe it or not, there are a "few" pilots who aren't quite as good as they think they are. It only takes ONE bad mistake to create an accident. Too many times in history, has there been a pilot who has said at the end, "I wish I hadn't done that". That's too late to admit you made a mistake.

Our policy is to leave the number of raw data and/or hand flown approaches up to the individual pilots. You don't have to be concerned with the abilities of our pilot force. We are one of the few carriers whose Ops Specs allow hand flown CAT II approaches. Our POI must have the same confidence in them as I do.

I'll be the first to admit that my flying skills are not what they used to be. But over the years, I've become a lot more knowledgeable and well rounded, and not tempted to do some of the things I did when I was full of **** and vinegar. (Including getting baited into an argument with you on these points).

>>If you're gonna take something out of my bag, you better have a damn good reason. Some better than "Because I said so".<<

Well, I guess we'd have a problem there. You'd always be welcome to come in and discuss our differences, but in the end I'm gonna win that battle.

26th Jul 2002, 05:31
It is one thing to have stated in the ops manual that forward slips in our fleet are prohibited, and another thing to give the reason why. I am very skeptical of companies that keep their decision making process hidden. I have flown for enough carriers to know that not all procedures were borne of enlightened knowledgeable professionals. When I was in fourth grade, the comment ‘because I said so’ held some credence; now such a statement is met with an acerbic demeaning comment. It is much more effective to state the reasons behind the policy, people then are more likely to be compliant. A statement such as ‘forward slips in our fleet are prohibited as it has been documented in Boeing study 249-7981 that such maneuvers lead to premature failure of the main fin spar.’

26th Jul 2002, 06:18
Every pilot needs a bag of tricks.


For the benefit of my curiosity when would you envisage needing this particular 'trick'?

Hew Jampton
26th Jul 2002, 14:13
Are you a clone of 411A, you certainly act like one? No, you can't be; while he too is often highly opinionated, at least he isn't rude and arrogant.
"What do we PROFESSIONAL pilots ...." - who are you excluding from this elite group, Dai Davies or me? You'd be wrong on both counts. If you want to hear it from an engineer: "...modern types of aircraft do not take very kindly to sideslipping. (Continues with reasons)" From Mechanics of Flight, A C Kermode CBE, MA, CEng, FRAeS.

You appear to want to concentrate on the airframe stress aspects and want to hear it from the lips of an engineer. You might well hear it from the lips of an engineer that a commercial aircraft is of course stressed for a 1g barrel roll (and it's possibly not prohibited in the 737 manual either), but should you do one?

Although you might be lucky, I doubt that you will hear from Boeing. I understand that for legal reasons Boeing will now only communicate with an individual or organization that has bought, or is actively seeking to buy, one of its products, and then will only comment on that specific product.

26th Jul 2002, 17:50
dvt - You have a very poor attitude.

1. Just because you created a post, you do not own it. Nor do you have the right to tell people not to reply to the post with whatever spin or retort they wish. Just because it doesn't serve the purpose that you wanted, the fact that it already has 3 pages of replies, means that the rest of us are interested. Sorry, old bean - this board is a discussion forum, not a mechanism for you to get specific answers to your questions.

2. You said "You know what. See ya! ....I'll email my concerns to Boeing.". Well.... erm.... why didn't you? Why the three subsequent replies??! What did Boeing say?

3. Accept others advice/experience. You don't know it all. There seems to be a pattern here. Everyone else is saying "erm... can't see why you'd need to... a professional should go around... not in my SOP.... specifically not authorised in my airline". Yet there's you, who seems to *think* you know better... ask yourself a question!

4. Suggest you take the matter up with your chief pilot. But be prepared for a dressing down.

I sincerely hope none of my family are ever pax in your care...

I bet you're the type who doesn't believe in CRM training, aren't you?

26th Jul 2002, 19:33
A telling difference between forward and side slips: Side slip by definition merely counteracts the crosswind component to allow a straight line of flight, but the useful forward slip is usually a much steeper banking turn counteracted by a much greater rudder deflection.

There is NO question that a forward slip will get you down faster than S-turns for an emergency. If turbulence is not a factor, controlability and loads might be within normal limits, but gust loads or a heavy foot will quite possibly put you far out of the surface loading limits for a big tail. Maybe also for pylon wobble on big wing-mounted fans. If the tail is one of the fiberglass type, the aircraft - in good conscience & with what we know now - may not be reuseable afterward.

26th Jul 2002, 19:56
dvt, the purpose of a thread is that it changes direction slightly with each new post (provided it's still relevent). I've yet to see an irrelevent post on this thread. A new (and old) 737 driver would probably benefit from the advice and cautions of other fellow professionals.

I try to hand fly / do raw data approaches on a regular basis WHERE APPROPRIATE, plus I have the opportunity to do visual / circling approaches. I agree hand flying is important, but I think these days CRM, Teamwork, and making sound desisions are almost more important. Very few accidents are caused by pilots not having the necessary flying skills, MANY, MANY are caused by bad management, breaking rules, and get-in-itis.

I know ATC can mess you up, but if you can't manage your approach profile to such an extent that you have to side-slip then I think you're in the realms of a rushed approach. Have you ever thought what the other seat would make of you slipping on finals? I think it would make him rather uncomfortable.
You should be able to recognise early enough that you use the gear, or ask for further vectoring, or you do an early re-position. If you're high enough so sideslip, there are other, better, flight manual options available, if you're too low, then GA.

I agree that its an interesting question about whether there is any reason why you can't slip, and I would like to know the answer, but I think it's your attitude that 'only cissies don't slip' which has alarmed so many.

26th Jul 2002, 22:28
YIKES ! 30yrs of flying transport aircraft here. That is one manuever I've never seen and hope I never do. I would
respectfully suggest you consider limiting forward slips to
light aircraft and gliders.

That swept wing airplane you are flying can and will do strange
things to you when you least expect it. Fly it like you were taught.:D, and it will treat you nicely.

bizjet pilot
27th Jul 2002, 15:16
dvt is a little irritating. There are two possibilities, it seems to me.

(1) dvt is confident of being right. I.e. it's all right to slip a 737-800 to lose altitude nearing a runway. In which case he just does it, and is confident of not having to explain his actions to a supervisor.

(2) dvt is not confident of being right. In which case he would be hostile and oversensitive in responding to the expressed reservations and concerns of most of the other posting members of PPRUNE.

Obviously (2) fits the data better than (1).

Now as to the merits of the quite interesting issues dvt raises.

(1) Nothing should interfere with the stabilized approach concept for largish jets. A slip (in such an airplane) is a not good idea below 1,500 feet. Surely that's a fair comment.

(2) There are slips and there are slips. Full deflection of a rudder are a really stupid idea in a swept wing jet. Light or moderate deflection with enough cross-control on ailerons, might be fine. Don't know. More to the point, neither does dvt. Is he comfortable with discussing "crossover" speed etc?

(3) If ATC gives you "crap" vectors, etc, presumably one has the ability to either dirty up or slow up. That way the "crap" vectors aren't as "crap" since the turn radius is tighter. Hence, no need for the heroics at the bottom of the approach.

(4) Many airline and jet pilots do aerobatics for fun. In an airplane designed for it. Treating a passenger jet differently (and more conservatively) is hardly effeminate.

(5) dvt needs to be counseled by his Professional Standards Committee. It's not his colleagues' job to justify their discomfort with slipping a transport jet. It's his job to justify doing it (in the absence of a compelling need to do it, or in the absence of an emergency).

(6) A Captain can do absolutely whatever needs doing in an emergency i.e. no engines, smoke in cockpit, etc. Slipping is one such thing. But when the aircraft is otherwise in fine shape, I just can't see slipping on a regular basis. That one's testosterone permits one to do it is not sufficient explanation.

28th Jul 2002, 05:37
For those of you a trifle reticent about “slipping” him bigfella balus, all B757/767/744 on an autopilot coupled approach, below 500 ft, handle cross wind by side/forward slipping ( take your choice) admittedly in the case of the B744 a combination of crab and side slip.

A B767 is all sideslip with the aircraft centreline aligned with the runway below 500’, in a max (A/P) crosswind the wing down is about 7 degrees, and it does it beautifully.

As for D.P Davies, with all due respect to his fans, I would strongly suggest you take some of his pronouncements with a grain of salt, in fact he had very limited experience on large aircraft, but that did nothing to moderate his views.

He also had a highly developed "not invented here" complex, that was not trans Atlantic limited, it also applied cross Channel. The Caravel was never certified in UK, ( the forward fuselages were originally built by Dh).

Furthermore, aircraft handling characteristics have improved greatly since the late 50’s and early 60’s, just fly your current party transport to it’s sops, not according to Dai Davies.

Quite frankly, some of the modification demanded to B707-300/320 on the G- register (non Boeing stab trim settings for T/O, modifications to the spoiler isolation system to solve a non problem, but making handling jam stab procedures far more difficult, stick pushers for aircraft that DID NOT deep stall, to name three) seriously degraded the safe handling of these aircraft. A stick pusher going off in the flare in strong gusty cross winds made for some interesting moments.

If my memory serves me correctly, for some time he would not even permit the use of Flap 50 for landing in the early BOAC 707. The claim was that the aircraft had a slight pitch up, before the nose pitched down, in a Flap 50 stall. This was quite true, it did, the slightest little nibble, but by that time the airframe buffet bordered on the frightening, there was no shortage of natural warning, quite apart from the fact that the stick shaker was also going crazy. But the fact remained, the nose always fell through.

I often wonder by how much we shortened the airframe lives of these aircraft in this kind of training ?? But it seemed like a good idea at the time. To my mind, given that any pilot doing this kind of type rating was already an experienced pilot, approaching stall buffet would have been enough, without doing full stalls.

AAAAAH !!, the Good Old Days. !!!

Tootle pip !!

Bally Heck
29th Jul 2002, 11:16
Hope I'm not being pedantic here Ledsled, but the B767 manual states "A/P systems initiate a slip with a maximum bank angle of 2 degrees when the crab angle exceeds 5 degrees"

There is a considerable difference between two degrees and seven. Also, the autopilot will be completely stabilised on the G/P and localiser, on speed and spooled up when executing it.

The other problem with sideslipping a swept wing aircraft is of course that the apparent airflow to the leading wing is much greater than it is to the trailing wing. If you have enough slip at low airspeed, I would imagine you could find yourself with an unexpected wingdrop.

Any expert comments on this?

Capt Claret
29th Jul 2002, 11:41
Referring to the link explaining the difference between a side slip and a forward slip, and to some of the posts trying to explain the difference, I can't see any difference, except that in the so called side slip, the amount of slip will usually be limited to that amount needed to off set drift. Whereas the so called forward slip is limited by rudder v aileron/roll spoiler authority and the size of the pilot's gonads.

The aircraft doesn't know or care which way the wind is blowing, both manoeuvres involve crossed controls and a flight path into the lower wing.

For my money, they're one in the same manoeuvre.

29th Jul 2002, 14:38
Bally Heck,

With all due respect, read a little further ---- at 500 agl, the A/P “kicks” off the drift, and from there to touchdown it is all sideslip, the numbers you have quoted are very close to what the 744 does under the same circumstances.

Would anybody like to comment on the L-1011 A/P coupled in a cross wind, I seem to recall it had a much higher A/P coupled limit than the 767 or the 744, which is 26 kt or so.

Tootle pip !!

Bally Heck
29th Jul 2002, 18:36

At the risk of going slightly off thread and also of repeating myself, I quote from the B 767 vol 2

The (runway align) submode operates as follows:

- actuated at 500 feet RA with LAND 3 or LAND 2 annunciated.

- activation not displayed

- A/P system initiates a slip with a maximum bank angle of 2 degrees when the crab angle exceeds 5 degrees

- Wing levelling from the slip is initiated when the ROLLOUT mode is engaged

ROLLOUT mode engages at 5 foot R/A so the aircraft will probably still be wing down at touchdown.

It does not at any time "kick" off the drift, and from the way I read it, a crab angle of much greater than 5 degrees would result from a strong crosswind once tha A/P had adjusted to 2 degrees bank angle. That however would probably happen at crosswinds outside the certificated limit for autoland.

30th Jul 2002, 00:22
Hey Gang,

Notch Johnson here. I've been away the past few days, with my girl BJ, slipping her other things besides airplanes. She likes to slip out to the country and ride the baloney pony with me. Anyways, I'm back and I'm truly astounded at the response of this thread. Though I think it has more to do with my charming personality than anything else. Right? I didn't want to discuss the merits of a simple flight maneuver, but you are right...I don't own this thread. So forgive me. Ok then.

Here's an excellent question on reader’s minds, submitted by Mouse...
"For the benefit of my curiosity when would you envisage needing this particular 'trick'?"

Well readers here my short response….”When going around is not the BEST option”.

I can hear you all now...."GASP! Oh the Horror! Murmur! Are you mad, man! Going around is ALWAYS the best option!" I’m my opinion; this is not always the case. Let me explain. It seems to me, many of you may only be familiar with flying into “flat and rolly" places like LHR. You can be forgiven for holding the view that Going Around is always superior to slipping it in. However, let me take you into some of the "Salad Bowls of Terrain" in places like Central/South America and the Caribbean. These places are scary enough in the daytime; try a visit at night. Many of these places have some of the most complicated and insane "Missed Approach" procedures you could imagine. Their "Engine-Out Missed Approach" procedures are even worse! You look at them and you say ..."You've got to be joking me! Right!" It looks like they were developed to satisfy some legal requirement knowing full well that Chuck Yeager might not be able to pull it off.

Anyways, case in point. I'm flying into one of these "Salad Bowls". I see that I'm high by a couple of thousand feet. I'm fully configured at final approach speed. There's a tailwind. My ND shows RED terrain all quadrants. So now I ask myself these questions. Can I “S” turn or 360...Hell NO! Shall I Go-Around and take a chance flying one of these widow-maker missed approaches...It depends. OR, can I perform a simple Forward Slip and be stable by 1000 ft AGL? Yes, I can. I would submit to you, given these circumstances, a Forward Slip can be a BETTER option than a Go-Around. Unless you want to give me a GREEN cockpit and a set of Night Vision Goggles, I think this maneuver is a good one to have in your "Bag of Tricks".

Don't get me wrong. I prefer a straight in same as anybody else. But I don't wish to debate the merits of a maneuver that has served me well, 5 or 6 times over the course of my career. It’s safe, it’s approved by the FAA, and it works. When someone tells me it’s unsafe, I get pissed cause I think it can save your @$$. One of the things I learned early on is " The PK (probability of kill) of a mountain is 100%”. Now, if you don't think you have the skills to do one, then don't. However, if you can’t do one of these, I don’t think your chances are much better on a “Window-Maker”. Put it this way…a simple Forward Slip is better than the potential CLUSTER FOOK waiting for you in the dark. And I’ll damn-well do again if I have too.

30th Jul 2002, 06:01
If you are "a couple of thousand feet high" then making the missed approach gradient would be a doddle, even with high terrain in the area - unless you began the approach at some massive overload.

Bally Heck
30th Jul 2002, 10:42
One assumes dvt that your company forgot to tick the "speedbrake" option when ordering their Boeings?

(Anyone else flummoxed by the "baloney pony" remark, or is he just being crude?):confused:

30th Jul 2002, 11:24
Bally Heck,
Are you quoting from a Boeing Manual, or a company version of a Boeing manual, that could be the difference, unless Boeing has changed things since I last operated B767.

As I said before, those numbers are very B744.

To those who were brought up on wings level/crab cross wind approaches, the B767 came as quite a shock to some, as if Boeing had committed some kind of heresy. As the manual will tell you elsewhere, you almost can't scrape a pod on B767, short of "shortening" a main gear, you will ding the nose or tail cone and a wing tip before you get a pod, quite a change from the "good old" 707.

Where I was working, very strong cross winds in severe clear conditions were common, there was no mistaking the behavior of the aeroplane.

Tootle pip !!

bugg smasher
31st Jul 2002, 04:21
Too right LeadSled, roaring forties mate, can’t handle a stiff crosswind, better go home and cry oily tears on mamma’s cotton apron lap.

So, dvt, you wanna a medal friend? Of course you can slip a large aeroplane, x-wind land or descend, all works the same, doesn’t take an idiot savant to put it together. Point is, friend, set her down on the first third, fuselage aligned with the centerline, on speed, that’s a good enough landing.

Anybody gotta serious a problem with that?

31st Jul 2002, 16:16
BS, a hearty I'm with you on this one!

If I remember the original query on this subject it was can you do it? I don't know, as I have never flown a Boing product. I do fly a Saab, however. The Saab manual specifically recommends a wing down landing, which if I understand anything means a forward slip technique (or else an ealiy arrival at the terminal). Maybe the Boing manual has a similar paragraph?

On the subject of 'slips'(aeronautical): as far as I can work itout there is little to choose aerodynamically between the foreward and the side versions, but do any of you remember slipping and sliding turns (not enough rudder/too much rudder)? You are all going to lose your medicals to ulcers if you don't relax a little.

bugg smasher
2nd Aug 2002, 01:11
And furthermore, tried it yesterday on an MD-11, no big deal. As far as I can tell, if you take into account airflow disruption to the downwind engine(s) and pitot/static ports by compensating with extra airspeed during the maneuver, it’s very benign stuff, my son’s kid sister could pull it off without wetting dvt’s drawers.

Can’t really believe the above garbage regarding fin sideloads (AA587 notwithstanding), surely the designers took into account the possibility of repeated heavy turbulence encounters and max-dem crosswind landings.

Also not really sure why Prof Davies aka Handling the Big Jets pens his cryptic warnings regarding this subject re large transport category airplanes. Anyone know if he’s still alive, willing, and able to provide a reasonable explanation? If not, surely the Cathay Old School Boys must know!? They know everything, bless their degenerate RAF hearts!!

2nd Aug 2002, 08:27
Bug Smash -- I readily concede you may be right - that slipping larger aircraft for glide slope enhancement just doesn't matter as an airframe life-limiting issue, but ONE detail is a source of concern:

Tail strength, etc are related during design to anticipated max crosswind speed for a given aircraft. However, no clear principle limits forward slips to the max angle implied by max crosswind speed. When slipping 'for effect', one tends to want to crank it in hard to get the 'feel' and then back off somewhat for tuning the result.

Done some distance above ground and at the pilot's discretion, a controllable fwd slip could quite possibly present an angle to the relative wind of TWICE what the max crosswind landing will do. Furthermore, this will be happening at velocities well above touchdown speed.

So, given that aerodynamic loads often increase at the cube of the airstream (vector) velocity, in fairly normal slip speedbraking one might be looking at 4x the tail load of a max allowed crosswind manoeuver - and that is before increments for turbulence and handling induced load factors.

4th Aug 2002, 21:51
In answer to the topic in question.....
Yes, the B737-800 can be forward slipped to increase descent rate on final approach but in 5000 hours of flying this aircraft I have only seen it done once and I am hoping that I never see it again. It is extremely uncomfortable - and not just for the passengers.
To end up in a situation where it would be verified is extremely unlikely and would be the result of incompetence on behalf of not only the ATC at the time but also both flight crewmembers.
Orbits on final have their own dangers (see the A320 crash in Bahrain whereby the circuit was performed with neither height loss or speed loss) however are alot more comfortable for aircraft and crew.
I am sure that Boeing recommendation for forward slipping as a way of increasing descent rate on finals would be the same as their recommendation for using speedbrake with flaps more than 15 or maintaining flap limit speed when extending flaps (using flaps as speedbrakes)...... i.e. NOT.

Do it once if you don't believe me. After doing so I am sure you will not do it again if your airline wants to keep their passengers.

bugg smasher
5th Aug 2002, 22:17
Point taken arcniz. Surely, however, the designers must take into account the possibility of a heavy turbulence encounter at high speed (something that would potentially generate stresses far higher than a max x-wind landing), violent yaw-damp failure induced dutch roll, or engine-out maneuvering at high power settings when considering maximum lateral loads to be withstood by the fin assembly.

redturk, I agree, the maneuver is not a pretty one, but for the purposes of understanding large aircraft behaviour and design limits, a very interesting one.

5th Aug 2002, 22:59
bugg smasher,

One ought to be VERY circumspect in presuming just what the design team does and does not consider.

The design standards, as frozen for the particular design, determine the minimum set of requirements for which compliance must be shown.

There are many areas where additional work would be very nice to have ..... whether the design group addresses such matters is difficult to determine. Having been involved in such things, I can only say that one ought to be a little cautious on the line ......

This is one of the reasons that we ought to operate, to the maximum extent reasonably practicable, in sensibly strict compatibility with the manufacturer's AFM and crew manual data.

It may be a bit too late, at the time, to discover that you have pushed the boundaries a little too far ...........

Cornish Jack
6th Aug 2002, 12:01
It's odd how the memory plays tricks on one - I would also have said, from memory, that the L1011 had a higher x-wind limit. Just dug out the Big Airways/Callie F.M. and the only reference to x-wind limits appears in the A/P engaged to 50' RH limits, which gives "reported wind not greater than 26kts, x-wind component not greater than 20 kts, tailwind not greater than 10 kts and no abnormal turbulence". For Tristar x-wind operation the forward slip was, of course, the ONLY way to do it and the other relevant entry in the F.M. is in the section on x-wind landings ...
"Touchdown on one main landing gear is permissible and recommended"

bugg smasher
9th Aug 2002, 15:59
j_t, if you were involved in aircraft design, perhaps you might be able to dig a bit and provide us with more information on fin design requirements. In addition to the above examples, I have been stuck on the ground in typhoon conditions in the Far East, and have seen from my hotel room window 747’s parked directly across the path of the oncoming storm. Those fins were subject to a sustained broadside (several hours) of 100 knots or more, with frequent gusts to 150, yet to my knowledge, those aircraft were not subject to any special inspection procedures afterwards. The only conclusion I can draw from this is that brick shathoose strength (technically speaking) is a basic design requirement for that particular aircraft structure.

Perhaps you can enlighten us with the facts.

9th Aug 2002, 16:30
Here's a link that gives some idea about the stresses Boeing engineers were thinking about when they design an aircraft's vertical fin...So long as you're not abrupt with th flight controls, a forward slip is well within the design limits. Which answers the intent of this post. Design limits can be exceeded when "Over Yaw" is induced by abrupt inputs to the rudder. What's far worse is to abruptly cycle the rudder in both directions. However, I think a Boeing would BEND but not BREAK with such inputs. Just a guess. But perhaps, Bugg Smasher, our resident test pilot will try this one on for us on his next trip.

It's reassuring to know that Boeing aircraft are designed to handle full rudder (steady state sideslip) in one direction, followed by full deflection in the opposite direction from steady state sideslip times a factor of 150%!!! That's one tough bird. What must be avoided is aburptness and the resulting "Over Yaw" that being abrupt creates. It would be Gameover on an Airbus. I believe an Airbus is designed to handle the stress of sideslip in one direction only.


bugg smasher
9th Aug 2002, 22:36
Excellent link dvt, a must read for everyone. As can be seen from the bulletin, the sideslip maneuver produces no unusual stresses on the tail whatsoever.

With that, I now stow my ‘resident test pilot’ wings safely in my desk drawer, and go out to drone along the night skies secure in the knowledge that if something is going to break, it’s not likely to be the fin. Anyone out there who wants to give multiple full rudder reversals a go, please make sure someone on the ground has a video camera.

10th Aug 2002, 02:35

Not an easy question to address and certainly one wherein generalisation may be more distracting than illuminating. While the present thread has, as its interest, fin loads, the following more general comments may be of some use ...

The Type Certificate Data Sheets will give an indication of the design standards which were applicable to a particular type. A review of the relevant issue (and the host of anciliary documentation) will give you an overview of the certification requirements applicable at the time ... Then there is the issue of design variations, concessions, equivalent safety determinations and the like .. Upshot is that, unless one is with the design organisation it is near impossible to find out just what was or wasn't done, especially in respect of matters extending beyond the minimum requirements. To a large extent the Industry relies on the professional integrity of the design organisation and the assessing regulatory body to give us some measure of protection .. and we have seen a few major screwups over the years where such reliance was proved to have been misplaced in the hard light of inservice experience.

It is important to realise that the people within the design and certification areas are just as human and fallible as everyone else. I have seen very competent engineers make assumptions which are quite reasonable to an engineer but which are materially at variance with what happens in the real world environment. Further, with modern design of large aircraft, as the projects become larger and discipline specialisations increasingly focussed .. there is the problem of overall design project management and what might slip through the cracks ....

To me the principal guiding light ought to be for the operator (and the operator's pilots) to follow the guidance which is laid down by the manufacturer (and ought to be in the operator's prescriptive documentation) .. for the simple reason that such guidance is based on the (usually unidentified) assumptions made in the design and certification processes. It really comes down to a matter of trying to walk through a minefield with marked safety pathways .. certainly you can step off the marked path and not have a problem ... or, conversely, do so and have a big problem ... difficulty is that you have no way of knowing outcome probabilities in the absence of data. It all comes down to a matter of sensible risk management and risk minimisation practices.

While being of sufficient modesty not to reproduce his comment here ... the earlier observation made by HOMER SIMPSONS LOVECHILD does, I think, sum up my thoughts precisely ....

My own worries tend to look less toward static strength problems (although the failure characteristics of composites can present some excitement) and more to the fatigue consequences of operation and maintenance practices which are at variance with the assumptions made within the original design and on-going MRB systems .. and we have seen a number of fatals over the years due to precisely this consideration. Having been involved in earlier lives as an engineer at both ends of the process I can only urge a conservative operation of the inservice article ... there are just too many imponderables and, as airframe lives are pushed out way beyond original life cycle projections, too many opportunities for spectacular outcomes.

In respect of your monsoonal observations, this is an example of the sort of situations which give rise to longer term cumulative structural concerns.

Scanning through the IFALPA/Boeing article in the link above suggests to me that Boeing, also, echoes HOMER SIMPSONS LOVECHILD's sentiments .....

This answer is probably not at all what you wanted ..... sorry about that... but, then again, if you have a TP background .. you know all too well all of what I have said above ...

10th Aug 2002, 13:42
I realise this thread started with reference to the B737-800, however Airbus has recently provided some timely information on the subject of JAR/FAR 25 certification of yaw control.
Basically, the most severe 'in service' encounter of lateral gusts, rolling maneouvre and asymmetrical engine failure must be taken into account. The aircraft structure must be able to sustain a design limit load of 1.5 without permanent deformation or failure over a period of 3 seconds.

There are further requirements/ considerations....

bugg smasher
10th Aug 2002, 14:31
"Boy, if the manufacturer don't recomend doin' something,dont go stikin' yer dick in there!! "

I agree j_t, with all of your comments; proceeding with great circumspection in light of potentially significant hidden and/or known design flaws, cumulative airframe stresses and faulty maintenance practices, contributes greatly to a pilot’s potential seniority. Re Homer Simpson’s comment quoted above, however, most manufacturers do in fact recommend slipping the aircraft, albeit in the context of a crosswind landing. The Boeing bulletin kindly linked by dvt indicates that this maneuver may be safely accomplished throughout the flight envelope.

In my view, therefore, if dvt wants to side-forward slip his 737 till the cows come home, I see no safety-related reason not to. That the passengers might view things from a different angle is a horse, of course, of an entirely different color.

10th Aug 2002, 15:50
Here's what Airbus says about their design considerations...


Basically, it seems they've designed their tail structures to the MINIMUM requirements specified by FAR/JAR 25. Boeing by contrast, has gone beyond the minimum requirements. I've always been partial to Boeings anyways. This is just one more reason to justify my prejudice.

However, Boeing points out, that while their tails are designed to handle a great deal of sideslip stress, the engine mounts are not designed to that same level. Cyclical over yaw, may overstress the mounts. Large sideslip angles, via abrupt inputs, may cause engine surge and stall at high power settings. Reason enough to be smooth on the flight controls at all times. This does not mean, that a forward slip can't be a smooth, controlled and useful manuever. I still see no reason to remove it from my "BAG". At least on Boeings. However, those of you flying Airbus' "made of glass", may want to keep your bank limiters set to 10 degrees and ask for long finals.

"If it ain't a BOEING, I ain't GOING."

10th Aug 2002, 17:33
Sorry dvt, didn't mean to step on your prejudices! For myself, I've had a choice for years as to which type to fly, but after sifting the B.S. have decided to eat my meals in comfort with a tray table and sidestick readily to hand.
However you do make a good and valid point that the Boeing is capable of rudder reversal from the steady state sideslip...
My reservations on slipping large, swept wing aircraft are the same as others have stated; largely concerned with control response, material fatigue, airmanship and comfort.
Dire circumstances notwithstanding, perhaps it is better to recognise the 'high and hot' situation earlier and do something more appropriate than resorting to side/forward-slipping. A go-around could be pro-active management of an unstable approach.

10th Aug 2002, 20:25

You do not provide details of the Airbus data to which you refer. However, your description suggests that you are referring to design static loads. Some problems relate to whether the prescribed loads are, in fact, the most severe or merely presumed extreme loads .. quite apart from the potential for dynamic responses giving rise to higher loads than are obtained by steady or smooth/steady control inputs ....

bugg smasher,

I have no problem with slipping .. quite clearly the manoeuvre is a necessary function of getting from A to B. My concern is only with some of the thread's comments which infer that ill-considered pilot control inputs do not present a potential problem ... as to whether the SLF might be adversely impressed by substantial slipping is another consideration altogether.


Again the problem (regardless of flight loads under consideration) relates to whether the design standard prescription is, or is not, sufficiently conservative to cover all reasonably expected real world situations. There have been instances in the past where the design standards have been found wanting and beefed up somewhat ... tailplane ice-related stalling, prescription of sharp-edged gust profiles, and earlier fatigue spectra assumptions come to mind, for instance. I wouldn't suggest that one ought to avoid slip manoeuvres .. merely that one ought to be conservatively cautious in large aeroplanes ... small trainers, such as the beloved SuperCub and like ilk on the other hand, are far more suited to heavy handed slip inputs.

As to the situation where the aircraft is held high or whatever ... if one cannot reconfigure to a higher drag and achieve a steeper profile at lower speed, then a precautionary early miss or orbit is probably the better option. My experience is limited to dinosaur Boeings .... there was no problem in achieving 1-1.5 nm/1000 when that was necessary ... provided that one was well ahead of the game plan and dirtied up early ... this might need to have been effected at 10,000-12,000 feet on occasion ... but it works just fine. The pilot who is just along for the ride and enjoys surprises, however, is an accident just waiting to happen sooner or later.

This has been a fun thread, though .....

12th Aug 2002, 16:35
People - inside aviation and otherwise - have grown more conscious of tail loads and airframe life issues after AA587. Metal fatigue and corrosion-related accidents in a maturing fleet amplify this awareness.

As J_T observes, the ability of an airframe to reach maximum service life is directly relates to how much abuse it has taken along the way.

I predict a move toward 'life-history' FDRs in nearly all future-build transport airframes, accumulating a 'perpetual' trail of g-loads and impulse events which may then be used to make refurb decisions along the way and then, finally, to determine the final flight date for the aircraft chassis. This use of LHFDRs will give 'tail-cover' for regulators, operators and insurers at a lower cost than the alternatives of 'excessive maintenance' or operation until a fatigue related accident/incident occurs.

Should make it MUCH harder to gloss over the barrel rolls.:p

Mud Skipper
12th Aug 2002, 20:58

I like the concept of life-history FDR's, it seems logical to not only track maintenance but also the loads the airframe has experienced over its life. For smoother operators this could reduce costs and increase resale and for those who perform such sideslip manouvers - well lets just say you would know what you were insuring or buying.


Why are you so agro that no one else supports the use of this technique. Did you do it on a check and are now having to answer to your company Gods?

I've had a fellow pilot select FLT and cross it up on final - hot and high, CRM non-existent - it's not fun for the other bloke. I know the aircraft can do it, but it ain't a good idea for more than just structural reasons.

If operating as a crew then you shouldn't surprise the other half, even if you do inform him of your intentions I doubt (time is limited unless you planned this from ToD?) he has time you consent or undertands to what extent you intend to do. It really sounds like you would be better flying solo.

Granted the Vnav data of late has gotten worse and worse but how do you miss manage your energy state so badly that this manouver is required. OK so your flying into a "Salad Bowl", big deal, on this occasion just be a bit more reserved.

Good luck with the company if they are trying to haul your ass.


13th Aug 2002, 02:57
Thanks Mudd Skipper.

I post facts. You post opinion. I'll take your opinion for what it cost me. Squat! If you had any real experience in Central/South America you'd have understood my post better.

For all intents and purposes this thread is finished. Boeing and Airbus have stated their position on the appropriate use of rudder. See my previous links. A forward slip is WELL within the envelop on a Boeing and within the enevelop on an Airbus.

Fini non?

Good Bye All.

Bally Heck
13th Aug 2002, 03:20
Bon Voyage dvt.

Now...... does anyone smell children...:rolleyes:

13th Aug 2002, 07:42
.. perhaps the sideslip issue has been well canvassed .. but, surely, the thread can move onto other new threads involving difficult approaches and ways and means of getting around the problems while still maintaining a high level of risk management ?

14th Aug 2002, 13:30
A quote from Air Publication 1979A, Cadets' Handbook of Elementary Flying Training !st Edition April 1943 - issued by command of the Air Council;

Page 85 chapter 14 entitled SIDESLIPPING.

"General Principles.
The manoeuvre known as sideslipping may be roughly described as making the aircraft descend through the air broadside on, while gliding. The direction in which the aircraft travels is at an angle to the direction in which the nose is pointing. We can sideslip in a turn as well as flying straight.
The advantage of the sideslip is that it permits us to increase our rate of descent, that is, to lose height more quickly, without increasing our forward speed.
By varying the amount of slip, we can vary the rate of descent......the degree to which any aircraft can be sidelipped depends on the extent to which the rudder is capable of overcoming the "weathercocking" of the aircraft, provided by the air pressure on the fin and keel surfaces This imposes a limit to the amount of bank which can usefully be applied and thus to the amount of sideslip....."

There is more - but by now you will have got the picture.

Reference to another old RAF manual of the World War 2 era, also has advice on sideslipping - but no trace can be found of the term "forward slipping".

Publication 129, the Royal Air Force Flying Training Manual - Part 1 Landplanes - dated November 1937 and reprinted June 1938, has this to say at Chapter 3 Basic Flying -paragraph 116 under the heading SIDESLIPPING.

"A sideslip means a state of equilibrium in which the aeroplane is travelling in a direction at an angle to that in which the nose is pointing. The unintentional slip is bad flying, but the intentional sideslip is a valuable manoeuvre to increase the rate of descent in a glide without gaining speed"....etc etc

So there you are. No such animal as a forward slip - not in those far gone days, anyway.

14th Aug 2002, 14:42
.. an Americanism, John .... I noted some FAA references in an earlier post ... if you like I can give you copies next week when I am back in town ....

Mud Skipper
15th Aug 2002, 08:20
Bring on life history DFR's.

It really is a brilliant idea. If there is one person operating foward slips and trying to justify it, then there is probably ten or more others doing the same.

No I have not flown in Central America, just a little country we call Papua New Guinea - short one way jet strips at several thousand feet and Wellington in NZ can be fun on good days too. Sorry not a pi*** contest but I would suggest things can become local SoP's but that doesn't mean they are good airmanship (whatever that means).

Personally I would not wish to fly or buy an aircraft which has been pushed harder than required, as an SoP. Fortunatly my ops generally buys new aircraft and we fly in a reasonable conservative world.


My best to the Kids:)

15th Aug 2002, 13:31
"Life history recorders" are a fact of life in many operations and will likely increase in use.

The devices are generally called quick access recorders or similar and provide a convenient means for the operator to monitor such things as performance, engine operation etc ... and operational exceedances. The former are a great boon to tech services engineering people ... while the last, with sensible management and union practices, can be a useful flight standards management tool.

Final 3 Greens
19th Aug 2002, 22:14
A forward slip is WELL within the envelop on a Boeing

Interesting ENVELOP????? Spelled twice this way in the post.

Now it's time for DVT to go back to MS Flt sim and practice his/her short field landings in the Concorde.

At least this line of rationale is reassuring to a PPL who travels as pax rather a lot and would feel very uncomfortable about anyone needing to slip an airliner, certainly whe he should be stabilised.