Old 10th Oct 2013, 08:20
  #345 (permalink)  
Owain Glyndwr
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 83
Posts: 474

But I understand that prudent formulation and choice of Davies, so long as pilot don't want to reckon some precisions... :
Iím really not sure what point you are trying to make here. If it is a plea for a mathematical definition of acceptable dutch roll characteristics then such things already exist and are used by engineers in the design stage to establish whether yaw dampers are needed (almost always they are) and to define the control laws under which they will operate. Search for lateral directional handling criteria and you will get a bucketful. (or MIL-F-8785C)
If you are looking for something to use in flight then I think that would not be very practical; besides which I am sure that pilots donít need a set of memorised numbers to tell them that the aircraft is doing something they donít like!

Why did Learjet not adopt Davies's method and stay wrong with the rudder use, and reversal use, and brutality use ?
I havenít a clue! Youíd have to ask the Learjet test pilots or whoever decided on that recovery technique.

Stopping the bad divergent oscillation is not all : probably your aircraft wants to play it again ! that divergence had a cause, and watching the motion quantatively helps to fly safe at home avoiding a new divergence with your jammed yaw damper...
I agree that in a divergent situation it may, probably will, happen again; but telling the pilot to watch the motion like a hawk to prevent build up of any roll oscillation is surely not the solution? In this regard prevention is better than cure.
Can we agree that a divergent dutch roll is only likely to build up if the aircraft is either naturally unstable in that mode or if the dutch roll is so lightly damped that inappropriate pilot inputs may drive it unstable? In either case the aircraft would be deficient against the certification requirements that call for it to be controllable without the use of exceptional piloting skills. Under modern certification rules (FAR 25.1309) the allowable probability of any system failure is linked to the consequences of that failure. So if yaw damper failure would result in a Hazardous situation, i.e. one :

which would reduce the capability of the aeroplane or the ability of the
crew to cope with adverse operating, conditions to the extent that there would be:
(i) A large reduction in safety margins or functional capabilities;
(ii) Physical distress or excessive workload such that the flight crew cannot be relied upon to perform their
tasks accurately or completely; or
(iii) Serious or fatal injury to a relatively small number of the occupants other than the flight crew.
then the failure probability would have to be Extremely Remote that is to say less probable than once in every 10^7 flight hours.
This being the case, I think that the introduction of quantified criteria for the pilots to monitor is not necessary.

The final object of certification is to do the aircraft safe for the wanted use. Today we have to increase air safety despite crashes statistics are getting better. All the crashes which can be avoided have to get progressively suppressed.
Well you wonít find anyone to disagree with those sentiments on this forum! But I have found over the years that if you have a problem it is best to attack the major contributors first. Today the major players in accident statistics by far are LOC and CFIT. The latter is being addressed and the rates are coming down, but Iím not so sure about LOC. There are a lot of things in there where better communication between the engineers and pilots would pay dividends, which is why I read with dismay some of the us vs them comments on these pages.
Even so, you would struggle to find any instance of an accident attributed to a divergent or neutral dutch roll characteristic. [I exclude the subject of this thread because the natural dutch roll was well damped (yaw dampers still operative) and because of the inappropriate control inputs]

Changing certification rules can perhaps help, and if it should help why not try ? Perhaps it could improve the speed of information from engineers to pilots ?
Alas! The one thing that has come out clearly in these discussions is the virtually complete exclusion of trained pilots from the certification rules. Until that blockage is removed there is, I fear, little chance that changing the rules would speed the passage of information.

(I know that you will teach me something there as you are a specialist of that beautiful adventure...)
Ooops! This is an anonymous forum

Last edited by Owain Glyndwr; 10th Oct 2013 at 08:25.
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