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Direct mode charachteristics and currency

Posted: Mon May 06, 2019 12:46 pm
by zuckie13
The accident with Aeroflot 1432 got me thinking about this.

It's been reported that the aircraft was in Direct Law after whatever happened in flight, so would have been that way for landing.

1) For FBW aircraft like the SSJ, or the Airbus fleet, how different is it flying the aircraft when in Direct Law compared to Normal Law? Obviously you lose most/all of the envelope protections that you would usually have, but outside of that how differently does the plane respond to control inputs when hand flying?

2) How much stick time do pilots get flying in Direct Law? I'd assume its rare to be in an actual aircraft and find yourself in Direct Law (I'd assume never with passengers on board except for an actual emergency), but presumably it is covered during simulator training?

Is there a risk that pilots may not have the hours needed to understand the "feel" of flying without all the protections and damping in place?

Re: Direct mode charachteristics and currency

Posted: Mon May 06, 2019 12:59 pm
by Starlionblue
All the following is from Airbus experience, not Superjet 100

1) Direct Law feels quite different. The aircraft is less stable. In Normal Law, you can let go of the stick and it will just remain on trajectory. In Normal Law it will constantly deviate, like a light prop perhaps. So you constantly have to correct. It is more work but quite controllable.

2) We get very little time in Direct Law. In the sim every now and then. That being said, it is an extremely unlikely situation. I don't think I know anyone who has actually been in direct law in flight.

I think pilots are trained to do what is needed to get the aircraft home in abnormal situations. Certification requires that an aircraft be controllable in any flight regime by pilots without exceptional skill. As mentioned direct law is more work, but it does not require superhuman ability.

Re: Direct mode charachteristics and currency

Posted: Wed May 08, 2019 1:29 am
by greendot
zuckie13 wrote:
The accident with Aeroflot 1432 got me thinking about this.

It's been reported that the aircraft was in Direct Law after whatever happened in flight, so would have been that way for landing.

1) For FBW aircraft like the SSJ, or the Airbus fleet, how different is it flying the aircraft when in Direct Law compared to Normal Law? Obviously you lose most/all of the envelope protections that you would usually have, but outside of that how differently does the plane respond to control inputs when hand flying?

2) How much stick time do pilots get flying in Direct Law? I'd assume its rare to be in an actual aircraft and find yourself in Direct Law (I'd assume never with passengers on board except for an actual emergency), but presumably it is covered during simulator training?

Is there a risk that pilots may not have the hours needed to understand the "feel" of flying without all the protections and damping in place?


In an Airbus, you have three primary laws: Normal Law, Alternate Law, and Direct Law. Normal law gives you the most amount of computer aided flight control surface management. It will prevent you from overstressing the airplane or doing something dumb, like banking more than 60 degrees or stalling the airplane. In Normal Law, if you get the airplane too slow, it will recover itself and climb away automatically. Alternate law has less of these "protections" but still has nice features like auto-trim and various energy warnings. Direct Law is nothing special -- it's just like a normal airplane at this point. You control flight surfaces directly. Airbus aircraft are a piece of cake to fly, even in Direct Law.

And yes, all these scenarios are addressed in simulator training. It's not that hard really. You're still just flying from Point A to B under benign maneuvering conditions. The difference between flying a FBW aircraft with flight control laws and something without any computer assistance is probably much more dramatic in military situations (e.g. F-18 v. F-14 in a dogfight or landing on a carrier).

Re: Direct mode charachteristics and currency

Posted: Wed May 08, 2019 5:16 am
by IADFCO
I realize it's probably never done, but if you wanted to go intentionally from Normal to Alternate to Direct Law in actual flight (not in a simulator), could you do it, i.e., is there a switch that a pilot could use?

Re: Direct mode charachteristics and currency

Posted: Wed May 08, 2019 5:43 am
by Starlionblue
IADFCO wrote:
I realize it's probably never done, but if you wanted to go intentionally from Normal to Alternate to Direct Law in actual flight (not in a simulator), could you do it, i.e., is there a switch that a pilot could use?


There is no switch per se, but you could force it by switching off certain combinations of systems, "simulating" malfunctions which trigger a reversion.

Apart from "doing it on purpose" in that manner, the only two reasons to end up in Alternate or Direct Law would be either if some malfunction triggers a reversion, or if some line in an ECAM/QRH procedure you have to perform triggers a reversion. For example on the A330, if you lose both Green and Blue hydraulic systems, or if the smoke procedure leads you to activate Emergency Electrical Config, you revert to Alternate Law. In the latter case, you are indeed making the flight controls revert "on purpose", but it is not the objective in itself. It is a side effect.

In a wider context, you never want a reversion unless it is forced upon you. The protections are there to help, and an abnormal situation is when you need them the most.

The controls act the same way regardless of actual flight or the simulator. For example, the gear won't extend at 300 knots in the sim, because it won't in the aircraft.

Re: Direct mode charachteristics and currency

Posted: Wed May 08, 2019 7:02 pm
by greendot
IADFCO wrote:
I realize it's probably never done, but if you wanted to go intentionally from Normal to Alternate to Direct Law in actual flight (not in a simulator), could you do it, i.e., is there a switch that a pilot could use?


Yes.

You turn off two ADRs. It turns a smart airplane into a 737. From there it flies just like any other airplane. It no longer prevents you from stalling or doing barrel rolls. Switching to "manual" is quite easy.

Re: Direct mode charachteristics and currency

Posted: Wed May 08, 2019 7:50 pm
by tu204
greendot wrote:
IADFCO wrote:
I realize it's probably never done, but if you wanted to go intentionally from Normal to Alternate to Direct Law in actual flight (not in a simulator), could you do it, i.e., is there a switch that a pilot could use?


Yes.

You turn off two ADRs. It turns a smart airplane into a 737. From there it flies just like any other airplane. It no longer prevents you from stalling or doing barrel rolls. Switching to "manual" is quite easy.


Then comes the question of what your company will do to you for that little stunt. :lol:

Re: Direct mode charachteristics and currency

Posted: Wed May 08, 2019 8:43 pm
by Woodreau
IADFCO wrote:
I realize it's probably never done, but if you wanted to go intentionally from Normal to Alternate to Direct Law in actual flight (not in a simulator), could you do it, i.e., is there a switch that a pilot could use?


It is done all the time. In order to get the flight simulator into alternate law or direct law, you have to disable systems to get the flight control law to degrade. It is the same systems you disable in the airplane to accomplish the same.

There is no alternate law button or direct law button.

To get the airplane into alternate law, you turn off two ADRs, from that point get into direct law, you lower the landing gear.

Re: Direct mode charachteristics and currency

Posted: Wed May 08, 2019 10:09 pm
by hivue
Starlionblue wrote:
the only two reasons to end up in Alternate or Direct Law would be either if some malfunction triggers a reversion, or if some line in an ECAM/QRH procedure you have to perform triggers a reversion.


For functional check flights following mx are Alternate and Direct law behavior never tested?

Re: Direct mode charachteristics and currency

Posted: Wed May 08, 2019 11:07 pm
by Starlionblue
hivue wrote:
Starlionblue wrote:
the only two reasons to end up in Alternate or Direct Law would be either if some malfunction triggers a reversion, or if some line in an ECAM/QRH procedure you have to perform triggers a reversion.


For functional check flights following mx are Alternate and Direct law behavior never tested?


Good question. I'm not sure but I don't think so because there's not much to test, as it were. The flight computers are on/off devices. Each individual one either works fine, or doesn't work and is locked out by the system. If a computer fails in flight, the procedure is to turn it off and then back on. If it doesn't come back fine, you turn it off and leave it off the rest of the flight.

Re: Direct mode charachteristics and currency

Posted: Thu May 09, 2019 3:31 am
by BoeingGuy
Starlionblue wrote:
IADFCO wrote:
I realize it's probably never done, but if you wanted to go intentionally from Normal to Alternate to Direct Law in actual flight (not in a simulator), could you do it, i.e., is there a switch that a pilot could use?


There is no switch per se, but you could force it by switching off certain combinations of systems, "simulating" malfunctions which trigger a reversion.

Apart from "doing it on purpose" in that manner, the only two reasons to end up in Alternate or Direct Law would be either if some malfunction triggers a reversion, or if some line in an ECAM/QRH procedure you have to perform triggers a reversion. For example on the A330, if you lose both Green and Blue hydraulic systems, or if the smoke procedure leads you to activate Emergency Electrical Config, you revert to Alternate Law. In the latter case, you are indeed making the flight controls revert "on purpose", but it is not the objective in itself. It is a side effect.

In a wider context, you never want a reversion unless it is forced upon you. The protections are there to help, and an abnormal situation is when you need them the most.

The controls act the same way regardless of actual flight or the simulator. For example, the gear won't extend at 300 knots in the sim, because it won't in the aircraft.


Boeing FBW airplanes have a switch to select Direct Mode, although there is no procedure to do so. You also cycle the switch if you attempt to up-mode from Secondary Mode to Normal Mode if your Air Data problem heal. (On the 787 you are pretty much locked into Secondary once you are there, but the 777X will allow going back to Normal in many conditions.). There are a number of 777X procedures that tell you to cycle the switch. When doing so, you actually momentarily go to Direct Mode.

There is no way to manually select Secondary Mode on the 777 and 787. They way they do it in flight test is to pull the Pitot Probe Heat circuit breakers.

Re: Direct mode charachteristics and currency

Posted: Fri May 10, 2019 3:32 am
by IADFCO
Many thanks to all of you who answered my question (#4). Very interesting answers, I learned a lot.

Re: Direct mode charachteristics and currency

Posted: Sat May 11, 2019 5:00 am
by 7BOEING7
Prior to delivery every 777 is flown in Secondary Mode -- by turning off both L & R Main generators and the L & R Backup generators. Once the power is recovered the PFC switch is selected to DISC (as noted above) to go to Direct mode. When done flying in Direct mode (it's no big deal -- that's what pilots are paid for) the PFC switch is selected to AUTO and the flight controls go back to Normal mode.