Heavy pull force required for rotation
 

Just reading an article about the Pilatus PC-24 jet which has a required pull force of 33 lbs. That sounds awfully heavy. I remember an ATR sim having a heavy pull force which was not a pleasant experience(but the aircraft was good).

Are there other aircraft like this? Boeings seem to have a nice feel. Airbus...…..take a guess.

Westwinds.

B737-300 FFS. 61 tonnes Flap One. Sea Level 15C. Stab trim correct at 5.2 units. Engine failure arranged at V1 with a VR 15 knots later. In other words a spread of 15 knots between V1 and VR.

In this simulator the stick force at VR on one engine is very heavy indeed and the control column is two thirds of the way back to rotate. It is common to see pilots struggling to reach iniitial climb attitude. If they relax momentarily during pull back, the main wheels hit the runway a couple of times before the pilot manages to drag the aircraft into the air. Stab trim is correctly set as part of the before takeoff checks. Once airborne and well clear of the ground, at least 3 units of back stab trim from original takeoff setting is needed for comfortable trim for single engine climb out..

I believe it is because instead of two upward vectors of thrust as happens with normal two engine takeoff, only one engine has its upwards vector meaning the stab trim setting is such that the aircraft is significantly nose heavy when rotating after the engine had failed at V1. But the VR does not occur until a few seconds later on reaching VR. By then the stab trim is incorrect at VR.

Most simulator exercises have a spread of maybe five knots between V1 and VR in the 737 Classic and by the time the engine fails the VR has already passed. Thus there is no chance of the very nose heavy trim manifesting itself. But throw in a case where there is a large spread between V1 and VR meaning a relatively long time on the runway accelerating on one engine, then this is a game changer.

. This nose heavy phenomenon only manifests itself if an engine fails while the aircraft is still on the runway. Apologies for rather scratchy description of this event in this simulator. Which brings up the question of takeoff stab trim setting. Does that setting mean the aircraft should be in trim at V2+15 all engines; but is invalid for single engine VR?

Next time there's some free time in the sim try a max weight engine seizure on a 737NG on FLAP 25 takeoff... A very different beast to it's normal simple self.

Interesting observation. If the handling difference is that significant, it would suggest the syllabus of training should include that particular sequence as an essential part of training; rather than only a "free time" fun exercise.

When EAL got their A300s the word in the sim was that the airplane did not handle as "badly" as the sim did on a V1 cut. I don't remember what the complaints about the sim were. But, within the year we had one lose an engine somewhere. The word came back, the airplane handles like the sim.

On rotation:

777-200 IGW GE90 - Just right.
777-200 ER Trent 895 - Waaayy out of trim, both feet on the instrument panel. Why? It’s a FBW aircraft... :confused:

Sims are fairly close to the real airplane but not exact. I think the sims are flown by CKA 2x(?) a year to verify that the 'feel' is similar to the real plane. Boeing 757 is the heaviest in the pitch. 767-300 is lightest. 737NG and 777 are somewhere in between. Heavy in pitch? I see 250 lbs (110 kg) guys flying with both hands and 105 lbs (50 kgs) women flying with one hand. Two hands is a unnecessary technique. Randomly it might be necessary, like a low altitude level off in a 757. The pitch force to counteract the thrust reduction is very high (30, 40, or more lbs of pull?)(tdracer might know the force).

Given that all the aircraft we’re discussing have powered flying controls, why is there such a discrepancy between loadings on different types (or even different variants of the same type)? Surely there must be an optimum level of feedback which allows the pilot to sense aerodynamic forces but allow the greatest accuracy when applying force to correct the flightpath? This is a universal interface problem which I presume was researched and answered a long time ago, so it leaves me scratching my head a bit.

It may be a gross generalisation but it would appear that most pilots prefer light to medium control forces over heavy ones and find those aircraft more easy/precise to control. I don’t think it’s a strength issue, more one of fidelity: if you’re applying large amounts of pressure to the controls you lose accuracy. So why deliberately build an aircraft that flies like this?

The amount of force isn't for a long period of time, that's what the trim is for. But on rotation the trim is set for takeoff power and V2. Do a de-rated, flex, assumed, takeoff? The force to rotate is higher. You're also below V2 which means the plane is out of trim by how much Vr is less than V2 (maybe 5-10 kts). The 757 pitch force going from full power takeoff to a 2,000' level off, where the power almost goes to idle, is very heavy. Smaller people might need two hands. Larger people don't but many use two hands.

Let's define 'heavy' - the 757 is, IMO, the 'heaviest' modern jetliner out there. But it's lighter than the MD-80 and 707. Maybe slightly heavier than the 727 (it's been a long time since I flew that). Part of that is the T.O. trim is set for full power, which is a lot in the 757 and has a noticeable nose up impact, but most takeoff's are done at standard/flex/assumed power. The A300 was the lightest. Small AB's probably come next followed by the 767's, than maybe the A330', then the 777's. The 737NG pitch is similar to what I recall the 727 - lighter than the 757 but heavier than the 767/777's and Airbus'. But in roll the 737NG is fairly light. All w/b's take slightly slower 'hands' (roll inputs) than n/b aircraft. It's a function of mass, wingspan, and large engines on the wings. W/B's aren't happy with rapid changes in roll rates or directions. The n/b aircraft don't shake as much when handled slightly poorly.

There is a lot of confusion on just what a simulator simulates. The TDMs produce a device that matches the OEM supplied data within defined tolerances and often this data is derived from engineering sources as opposed to real flight test data. The OEMs maintain that this engineering data is sufficient but there are many within the industry (including regulators) that do not totally concur with this approach!

Likewise the annual requalification checks by the regulatory authorities are equally misunderstood. The authorities are not there to "verify that the 'feel' is similar to the real plane" but to check the operators Compliance Monitoring System. Within the CMS there should be (typically) monthlu QTG checks, subjective fly out programs etc.. all to verify that the simulator still conforms to the test results from its initial qualification. In fact, there is no regulatory requirement for the authorities flight inspector to even be rated on that aircraft type and often he/she isn't!! (The NAAs rely on the operators to provide a type rated pilot to assist them during the evaluation.)

Having said this, control forces should be very representative of the aircraft being simulated and if they aren't then this should be written up for the operator to investigate and correct.

Lighter control feel, not weight.

I'm guessing the 105lb woman had her brain engaged into the flight control loop - anticipating what forces will arise and when, using proactive trim, making thrust changes at times so as to have the pitching moment be helpful rather than working against her, etc. Nothing preventing the 250lb guy from doing the same, but he doesn't have to, as he can get by gorilla-ing it without worrying about a thing - "they don't pay me any extra to handfly it" :yuk:

Well for the "variants of the same type" part of the question, it's been noted that different engine installations change the behavior. Also aerodynamic differences like different stretches would change the length of the lever arm of the elevator, with obvious implications. And I don't see the manufacturers spending the money to make changes to compensate the felt control force, unless absolutely necessary, i.e., the latest version doesn't meet certification standards, and has to be brought back within them.

Anyway, there's at least one poster on this forum whose name shows why he's supremely qualified to hit this one head on and in detail - and I'm eager to see his answer!