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Forward Slipping a 737-800

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Forward Slipping a 737-800

Old 23rd Jul 2002, 17:12
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dvt
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Forward Slipping a 737-800

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?
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Old 23rd Jul 2002, 18:28
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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?
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Old 23rd Jul 2002, 21:33
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dvt
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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.
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Old 23rd Jul 2002, 21:48
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Q: in a jet, does a slip lead to compressor stalls?
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Old 23rd Jul 2002, 23:43
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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
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Old 24th Jul 2002, 07:12
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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...
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Old 24th Jul 2002, 07:49
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If things are that desparate why not go-around?
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Old 24th Jul 2002, 13:30
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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'.
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Old 24th Jul 2002, 16:27
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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.
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Old 24th Jul 2002, 16:49
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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.
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Old 24th Jul 2002, 16:50
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I wouldn't try S-turns in a congested area.
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Old 24th Jul 2002, 18:28
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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.
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Old 24th Jul 2002, 19:08
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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!
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Old 24th Jul 2002, 19:37
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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 ?
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Old 24th Jul 2002, 19:49
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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.
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Old 24th Jul 2002, 21:20
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Excuse my ignorance I know what a 'side slip' is but what is a forward slip?
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Old 24th Jul 2002, 21:28
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http://www.bayareapilot.com/forward_slip.htm
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Old 24th Jul 2002, 22:17
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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.

P77

PS By the way, I lost my flight director once and didn't "**** my pants", but landed uneventful.
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Old 25th Jul 2002, 08:58
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All,
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 !!
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Old 25th Jul 2002, 11:14
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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 ....

Last edited by john_tullamarine; 25th Jul 2002 at 11:24.
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