View Full Version : Dive and Drive?
RatherBeFlying
16th Mar 2004, 02:34
Finger trouble made this a new thread rather than an addendum to the Bassersdorf thread where the a/c followed the wrong glideslope into the ground.
At the risk of being declared behind the times, my feeling is that dive and drive is harder to screw up bigtime than cooking up a glideslope, especially after a long duty day.
I can't get rid of the feeling that vertical speed was selected (perhaps by faulty and fatigued memory) to conform to the charted glideslope.
A foolproof way to set up a glideslope from FAF to VDP would be a better deal -- and less likely to hit something in the case of an MDA bust.
I suspect it could be done today with VNAV and a bunch of fancy button pushing.
Airbubba
16th Mar 2004, 03:51
>>At the risk of being declared behind the times, my feeling is that dive and drive is harder to screw up bigtime than cooking up a glideslope, especially after a long duty day.<
I think you're talking round dials but your point is also perhaps valid with glass.
Some of the nifty "build the glideslope" FMS techniques are great in the sim but a real soup sandwich if you miss a step or forget to enter a constraint. Some of the Boeings have a bad habit of entering go around mode at MDA (or whatever it's called these days) if you forget to reenter the runway in the box at the right place when building the approach manually. You see the runway, pop off the autopilot to land and the plane thinks it's doing the missed so the autothrottles go full bore (well, almost) just when you think you have the landing made. If the autothrottles were popped off with the autopilot, they miraculously reengage just when you don't want them.
Fortunately, more and more non-precision approaches are already in the box with VDP's and pseudo glideslopes these days.
Wiley
16th Mar 2004, 04:41
“Dive and drive” might be a great idea in a lightie searching for a break in the cloud base, but IMHO, it’s a very bad one in anything bigger. For all sorts of reasons, I’m in the “cook up a glideslope” camp – but I believe the glideslope, (indeed, the entire approach), should definitely “cooked up” and proven by a Nav Department on the ground, and definitely not by a crew “on the fly”.
Just my opinion.
Exactly, Airbubba. See F-GITA accident.
From an earlier post of mine:
.....There was no FCU malfunction, the overrun occured because the crew didn't know that at the end of the VNAV descent, which coincided roghly with the MAP, the autothrottle would have commanded go around thrust (incredibly, during the investigation, they found out all other AF 744 pilots didn't know as well this behaviour).
The copilot, when he felt the thrust levers advancing, instead of disconnecting pulled them back and hold them back (!), yes, all except N°1 (small hand maybe...).
At touchdown of course there was no spoiler nor autobrake, nor the captain was aware of what was going on.
Despite heavy braking, eng 1 was still producing full thrust when they skid to the right into the lagoon...
Firefighters eventually shut the engine down with water...
F-GITA was returned to service, and the NTSB recommended the E&E bay shouldn't be positioned there in the next millennium
RatherBeFlying, my modest opinion as posted some months ago:
"DIVE & DRIVE IS SAFER!
Yes Dive&Drive is safer.
And also much easier.
Those who find the level segment destabilizing simply have lost all their handling skills and try to hide behind the constant angle magical solution (which doesn't work, and puts them in an even worst and dangerous situation).
They try to turn a NON precision approach into a PRECISION one, simply because they don't know how to fly it and are a bit scared of it.
These are the pilots who crash in the non precision approach, the dangerous **** of Guam and many other places!
Ok, let's cool down and try to explain.
Basically, there are two kinds of NPA: the easy ones and the difficult ones.
That means: the ones with various altitude vs distance check points reported on the plate, and those without anything.
These can be flown in good weather or bad weather.
When I say bad weather I mean ceiling just above the MDA. Poor visibility, of course.
Avionics can be advanced or old.
AP ON or raw data manual flying.
In an argument like this we must assume the worst scenario and examine it.
No doubt once we master the most difficult scenario we can master the easy one.
So the scenario is: NDB over the field, and nothing else!
Outbound leg of, say, 3 minutes, turn back inbound and descend to the MDA.
MAP over the beacon.
Those who plan for a constant descent will calculate a certain VS versus a certain speed, trying to correct for the wind.
They think they'll be able to reach VDP (Visual Descent Point, the point at which you can leave MDA on a 3° slope to the runway) using the calculated parameters.
Very funny!
Actually, I'll tell you what will happen:
First of all, when you overfly the NDB outbound your timing will be inaccurate, because of the dead cone and the instrument tolerances, and I've noticed this error can reach one mile (!);
Second, all your meticolous calculations will be falsified by the real wind, which is different from what you had expected at different altitudes throughout the approach; also, because of the difficulty involved in maintaining exactly the speed and VS you had planned.
The result will be: you'll certainly NOT reach MDA at the VDP, and what is worst you won't know if you are high or low on your imaginary profile.
In both cases, a missed approach is very likely, since you will end up too far out, or too close and high.
All these unnecessary goarounds reduce safety (more time spent in the air thus more chances of failures, more stress and pressure with low fuel and so on...).
But now, Pittsle, you are with me in the cockpit:
we'll fly this approach using the Dive&Drive method.
With this technique, it's not even necessary to make any meticolous calculations, as we'll know for sure on which side of the imaginary glide we are, so we'll know exactly what to expect and what to do.
Leaving each altitude step you will initially set about 1500ft/min, then approaching the selected altitude you'll reduce to about 1000ft/min.
I say about: it does't matter if your VS is not exactly a certain value.
This will certainly NOT trigger any GPWS warning.
You'll realize soon how easier it is to concentrate on one thing at a time: descending to your next altitude and levelling off, till the next one, instead of worrying all the time if your VS is still correct, if you haven't screwed it up, and if you'll reach the next step high or low...
Just one figure in your mind: the altitude you are descending to.
We'll have the Landing cklist completed down to flaps 15 on the 737 (flaps 10 if single eng.) before turning on final.
Once established on final you'll descend to the MDA the same way: there again, you'll notice how easy and safe it is to descend worrying only about one thing: levelling off at the MDA.
You have certainly noticed how difficult it is for your friends who use the other philosophy to control the path of the airplane and make a decision at MDA: they have to fly a descending airplane, monitor the altitude not to bust the minima, look outside for the runway which can be offset by a large amount in a non prec. approach, find it, assess their height in relation to the PAPI to decide about the feasibility of the landing, ALL THAT SIMULTANEOUSLY.
Not so in our cockpit: after a nice and smooth leveloff at MDA, the airplane will be stabilised, we'll know for sure on which side of the profile we are, and we'll have plenty of time to set the Missed approach altitude in the MCP, look for the runway, find it, and make our decision.
Another important point to consider in bad weather: if the ceiling is roughly coincident with your MDA, if you descend with a constant rate and plan to decide at MDA, one little limb of the cloud is sufficient to keep you in IMC and force you to goaround! while with a longer level segment at MDA you have many more chances to exit the cloud and become VMC.
Ok, Pittsle, now we are approaching the VDP and have the runway in sight: as soon as the PAPI becomes three red one white, you call flaps 30, set Vat, and leave the minima on the PAPI and land.
If the minima is lower than 500ft, you have two options:
you can plan a flaps15 landing, or you can arbitraryly set MDA=500ft.
This way you ARE configured by 500ft.
Summing up: you know exactly what is happening instead of hoping for a miracle to end up exactly at the VDP at MDA, you know what to do, much easier job thus safer job, and what is a sort of nightmare to more than one pilot becomes very easy and effective.
Once again, if you master this scenario, you can apply the same rules to an easy one (as described at the beginning) or whatever else you like, even the constant descent, of course if you are in CAVOK even my dog can fly a nice touristic approach
One last pearl of wisdom : busting the minima by 30 or 50 ft, although very very ugly, has never killed anybody.
Being flown by the airplane, instead of flying it, like the Guam ****, has.
LEM
The VNAV approach profiles have been out for only a short time. There are many restrictions disigned to prevent errors until all is sorted out. It seems if one stays within the operating limitations imposed by company, it is a better way to fly a non-precision approach.
The details of why the VNAV or autothrottles do something, or do not do something is available. It is mandatory reading at my company.
Airbubba wrote, "Some of the Boeings have a bad habit of entering go around mode at MDA (or whatever it's called these days) if you forget to reenter the runway in the box at the right place when building the approach manually."
I'm not allowed to build or change fixes that are located inside the FAF. However, it is understood that if I get to the EOD (End of Descent Point) and the next fix is the missed approach holding fix, I am certain that the autothrottles will reposition for the missed approach. That is the next phase of flight programmed in the FMC for this procedure. It is more likely that the EOD is the runway. In either case, this is plain to see on the legs page.
As time goes on, and these VNAV approaches are practiced at different locations, a better understanding of "what's it doing now" can be attained. It has been difficult to leave the dive & drive that has worked so well for so long. I am willing to give VNAV a shot, and I am willing to spend the time improving my understanding of all of the pieces of the system.
Hmmm,
If you don't absolutely KNOW 'what its doing now' then you had better stick with dive and drive...a long time proven technique, even in heavy jets.
IF you know how, that is..........
With the exception of FMC failure, unusual approach procedure, or Captains decision that the VNAV cannot be accomplished for a particular approach, the dive and drive is not an approved procedure in the Ops Specs.
The learning points since initial training on these procedures have been for unusual approaches, like the JFK Canarsie VOR 13L/R...now recommended using vertical speed only.
Max Angle
17th Mar 2004, 11:20
Imagine yourself standing in a muddy field about 2 miles short of a poorly lit runway, the wind howls, it's raining hard out of a low overcast and is all in all a real crappy night. You hear the scream of jet engines at high power, a heavy 4-engine jet passes overhead at 350ft, flying level with full flaps and gear down, lights blazing and rocking and rolling in the turbulence as the crew search for the runway and wait for the MAP to come up. Does dive and drive still seem like a good idea?, not to me it doesn't.
RatherBeFlying
17th Mar 2004, 14:10
If the glideslope and VDP/EOD... are included in the approach installed in the FMS and come up automatically when the approach is selected, I believe most pilots would prefer to use it.
Manual entry of waypoints and altitude constraints while on the Initial Approach segment seems rather late for this kind of fancy work. While manual entry might have been done and crosschecked properly before beginning descent from cruise, if the approach gets changed in the terminal environment, dive and drive seems the less errorprone method in a high workload phase.
Max Angle,
IF you are at 350agl in your 4-engine heavy jet, as described, without the runway IN sight, you would be well below the minima for a category D aircraft, and would be quite illegal... not a good thing.
Max Angle
17th Mar 2004, 20:37
Well I was just making a point, call it 500ft then. Still seems like a pretty poor idea.
Some people in the African forum, used to fly a 60 tons airplane for hours at 100ft to avoid a SAM, would definitely disagree...
;)
Max Angle,
Yes, would agree, it is a rather inappropriate maneuver, but sometimes necessary to get into some 'lesser-developed' airfields or...developed ones after certain hours (hello ZRH).
SaudiArabian had fitted to their TriStars the latest and greatest (at the time) FMS, which was built by Litton, IIRC (circa 1979).
Full area nav capability, full performance/thrust management (all regimes of flight, except takeoff) and, it also had full vertical nav capability as well. The VNAV for descent could be pilot programmed for non precision approaches, altho it was not so certified.
Recall one Captain, who was very adapt at this programming until one day getting up close and personal with a hill just south of Madina...strangly, he lost interest after that...:uhoh:
greybeard
22nd Mar 2004, 11:04
Gidday,
Dive and Drive or Stable descent we will never all agree and every place can have a different need.
The ORIGINAL concept of non precision approaches was, in Aust anyway, NEVER aligned with the runway so the D&D proceedure was the way to go so MAX time could be spent at the Minima to assess the Vis and do the circling approach, with the min at or above the circling.
Then along came the RUNWAY NPA, which led most if not all to consider a stable sink rate to arrive at the minima and if visual go ahead and land.
Good stuff if the minima is on the correct slope and reasonably aligned with the runway.
Smaller A/C can re-enter the descent from level flight relatively quickly. BIG ones tend to follow the level flight path for a while which CAN put you high on slope needing high sink, rather too close to the groung for common sense.
The DME or GPS distance can assist in orientation for all this but there are still places where it can go pearshaped really quickly such as Kathmandu and others I have not had the privilage of attempting.
Be properly briefed, prepared, SOPs well known, be really careful and all will be usually OK.
Remember-- GRAVITY SUCKS
:ok: