>Because jumping to conclusions is not helpful. Let the NTSB do their job and find out what went wrong
We are used from previous cases that aircrafts will get grounded until we are sure the airplane is not at fault.
So why in the past this happened before the investigation was complete but in this case the risk is considered t be minimal? I am not saying that it is right or wrong but I want to understand the algorithm that is used to decide what to do in this cases.
We are used from previous cases that aircrafts will get grounded until we are sure the airplane is not at fault.
To be clear: you're suggesting that grounding an entire type of aircraft, either worldwide or even just within US airspace, is a common response to accidents?
I ask because the last general grounding of a type was in 2013 to remedy the 787 battery issues. Prior to that, the last such grounding was the DC-10 in 1979.
Airlines can voluntarily ground their planes. Manufacturers can recommend it. Generally, only a government agency can force it and they only do so in truly extreme circumstances (often requiring multiple serious incidents). More commonly in the US, a directive will be issued to airlines telling them to carry out some remedy (fix/replace a part, change a procedure, etc.) and a window of time in which to complete it. They don't speculatively ground entire air fleets every time there's a potential problem.
For example, in 1991 a Boeing 737-200 crashed in Colorado with the loss of all lives on board. In 1994 there was another crash of a 737, in Pennsylvania, again killing everyone on board. In both accidents the investigation pointed to unexpected movement of the rudder.
Several other cases were identified where pilots claimed the rudder was moving unexpectedly, but they were able to land safely.
The follow-up took several years of investigative work, and eventually identified a situation where a servo could jam and deflect the rudder.
Despite two severe crashes and many other suspected non-fatal incidents, that generation of 737s was not grounded; the remedy, once the cause had been determined, was an order from the FAA to replace the servos, with a timeline by which the replacement had to be completed.
I'm almost certain that the solution in this case will be training. Almost exactly as you've said in another comment. The correct response to an commanded stabiliser movement is to set the stab trim switch to cutoff. Pilots will probably experience this failure mode in their next recurrent training.
The accident is more than likely to be attributed to a system being added without sufficient guidance being added to the PoH, some degree of maintenance failing on behalf of the airline and finally a failure of the pilots to identify an issue and take appropriate action.
It's unlikely that those particular holes in the cheese will line up again in the near term given the emergency AD. No reason to ground the fleet.
That does seem likely, but very often the eventual cause discovered by the NTSB was not what people initially thought it was.
It's still worthwhile to warn the pilots about uncommanded movement and reiterate what to do about it. At this point I doubt any 737 pilot doesn't know about this.
To add to your summation, I'd expect the mechanism's inability to detect a faulty angle-of-attack sensor needs addressing.
We are used from previous cases that aircrafts will get grounded until we are sure the airplane is not at fault. So why in the past this happened before the investigation was complete but in this case the risk is considered t be minimal? I am not saying that it is right or wrong but I want to understand the algorithm that is used to decide what to do in this cases.