Posts by g5flyer

    Do flight simulators adequately reproduce 'ground effect'? That cushion that landing commercial aircraft seem to sit on for a few seconds prior to touchdown. In forty years of working in a control tower, I've seen every type of landing. Short, long, floated, bounced, bounced twice, touch-and-go, over-pitched, wheelbarrow and crosswind with wing-down or drifted. In flight sims I've probably done them all but I cannot seem to simulate a float with ground effect.

    The most common ways to get into a float situation is to be fast or a flare more than the typical 2 to 3 degree pitch change. The text book flare mainly diminish the sink rate and the plane is flown onto the runway. There are some circumstances that could put you in a float situation.


    1. The winds are pretty gusty and you are carry your gust additives up to 20 knots max on approach. The manuals have rules such as, half of the steady state winds above 10 knots plus all of the gust up to 20 knots. Each aircraft has some minor differences on how you add gust additives. We laugh in the gulfstreams because they tell you to bleed your additives crossing the threshold. In the G550, we pull power to idle at 100ft and drive her on in. They bleed speed slowly and you might lose 5 knots by the flare point, but that's it.


    2. You are flying a heavy type aircraft with large wing engines and the approach and landing speeds are held high at light weights to ensure you have air minimum control speed in the event you go around single engine and use all of that thrust. In this case, again you may end up in a little float. The 777 with it's large engines will keep the approach speed higher at those light weights.


    3. Lastly, you are just having a bad day and ended up fast on the approach because you were too high and couldn't bleed the speed or you raised the nose to 4 to 5 degrees in the flare and prolonged.


    Mainly, if you are faster than the normal approach speed for your weight, you will float with a normal flare. There are plenty runway over runs because a jet was fast, entered the flare, floated midway down the runway before touching down and not being able to stop. I have also seen where a jet is too fast, touched down, but the aircraft did not settle well enough. The ground spoilers would not deploy and the reversers could not be deployed. The plane was so fast that the weight on wheels switches could not be made allowing spoilers and reversers. With out the weight on the wheels, the brakes are not as effective.

    In the AFM, you won't find a section that define good or bad landings. But, in the limitation section and emergency procedures/abnormals section, they do define the line for hard landings. In every jet I've flown through out my career, a landing at 600fpm or more required a hard landing inspection. If over max landing weight, 600fpm is lowered to 360fpm. I always considered that an odd number. The manual would also address bounced landings which basically said re-establish the landing attitude.


    To the original question, yes, prolonged flare can lead to a hard landing, but more likely a tail strike. The landing geometry consists of crossing the threshold on glide path and airspeed. Initiate the flare at the proper flare height and touch down no less than VREF - 5. This will happen with a normal approach and flare. Once you move outside of those parameters, you are subject to risks. The problem with a prolonged flare is that you allow the airspeed to decay and a higher than normal pitch is established. At some point, the aircraft runs out of airspeed and the aircraft drops onto the runway instead of being flown on to it. Touching down less than VREF - 10 with higher than normal pitch angle certainly almost guarantees a tail strike. If your flare was high enough, you could indeed exceed the manufacturers max landing fpm as it drops to the runway. On the other side of the coin, your prolonged flare could result from being faster than required for the approach. In this case, you waste runway and risk an over run. Guy once told me while flying a leg, the most worthless blocks of concrete are the ones already behind you and the ones beneath you.

    Thanks Jet-Pack!,

    I did notice that aircraft tend to auto flare. Must be the effect you describe as the nose up tendency. If you guys ever need me to test things or have a question regarding aircraft behavior or performance, don't hesitate to hit me up. I've done a lot of check flights/shake downs for aircraft coming out of heavy depot maintenance. It's actually fun taking an aircraft up and putting her through her paces. You learn a lot about the aircraft and what it can. We do at all from accel time, stalls for stall system calibration and battery only ops to flight control checks and landing performance.


    Rick

    Greetings All,

    I am looking for a way to tweak roll/pitch response and ground effect in some of the jets. I fly real world jets and I have light, medium and heavy experience. I am current and qualified in the Gulfstream 5/550. I am looking to recreate the feel and ground effect I experience when flying real world. Though this is also impacted by sim control setup, I find some jets, like the Lear 45 to be just a tad bit sensitive in pitch and roll. In most jets I've flown, the yoke is rigid due to artificial feel springs and cartridges.


    Roll and pitch rates are more sluggish during takeoff and approach/landing. In other words, the yoke has a heavy feel to it.

    The aircraft is stable/more resistant to input. On approach in gusty winds, you will find yourself doing what I call yoke cardio. You will have to put in large inputs to counter the aircraft's rolling and pitching moments. In essence, there is a slight delay in control response. Flying straight and level, I can quickly put in 45 degrees of yoke left and right and the plane will hardly move. This is why on gusty/bumpy approaches, you will see a lot of quick yoke movements and little aircraft response. I've demonstrated this to new guys during air refueling. They would close in on the tanker furiously pumping the yoke all around and back and forth. By the time they are in the contact position and plugged, they are so wore out from cardio that they fall off the boom quickly. That's when I take the jet. Back out to 300ft. I then have them watch and rock the yoke 45 degrees left and right and show them that the jet barely moves. I tell them the jet takes time to respond. I drop my armrests and demonstrate how I can close on the tanker using finger tips and wrist. I use small inputs, leaving them in until the jet responds and adjust. I fly to the contact position without yoke pumping cardio/PIO. So I am looking for a way to adjust the pitch and roll input in the flight model to simulate more stability/resistance to input.


    Last thing I would like to adjust is the ground effect. In a nut shell, ground effect is about a 200 to 300 feet per minute decrease of sink rate in jets. Should start happening at half of the jet's wing span. So for me, 93.6ft is my wingspan, effect starts around 45ft. So lets look at the typical approach in a jet(MD80/737). Normally, you are looking at a sink of about 700 to 800 feet per minute on a 3 degree glide. The text book procedure, similar to most, is to start a flare at the 20ft call out. At 20ft, you pull power to reach idle by wheel touch down. You add 2 to 3 degrees to your current pitch and hold it through the flare. The aircraft will touch down close to your aimpoint about 1000ft down the runway. Here is the approach with ground effect. The aircraft is trimmed to the approach with a natural tendency to stay on glide path requiring slight pitch corrections. As you start to enter ground effect, the nose will want to drop, requiring some slight back pressure on the yoke. If your aircraft's procedure is to cross the threshold at VREF, you will require a little more back pressure as the speed you are carrying bleeds off(I.E. bleeding off wind additives). For each 5 knots of speed change, the aircraft pitch will change about 1 degree to maintain glide path. At your flare height, you add in 2 to 3 degrees to your current pitch and allow the jet to fly onto the runway. As you add your flare input, that back pressure will be maintained through touch down. You may have to add just a little tad more to maintain your attitude because the jet is losing speed in the flare. You start your power pull at the same flare height and should reach idle by the time the mains touch. Your touch down should be 1000ft down, possibly 1200 to 1500 if you are one of those fight for greaser types. Of course, long bodied aircraft such as heavies will touch 1200 to 1500 down the runway due to using long body VASI/PAPI(2 pinks/2 whites) and higher flare height.

    In the past, I did adjust the left/right aerowing ground effect factor/scale. There seems to be 2 sections containing these and i'm unsure of which of the 2 sections to adjust.


    I know these adjustments are not for everyone, but I love when I get into a sim and the aircraft has the same feel and effect that I experience in the real aircraft. I feel right at home. Your help is appreciated.


    Rick

    I can't speak for Airbuses, never flew them. Aircraft with low wing engines become problematic when the autothrottle starts to cycle. In the DC10, the throttles would go to full power or full idle if you were off selected speed by 5 KTS. As the engines surge back and forth, you are constantly fighting the pitching moment induced. So when hand flying, you end up working harder. For this reason, we had techniques such as 10% of your gross weight plus 23 for power. If I was 400K, 40 + 23 is 63% N1 for a 3 degree glide with flaps 35. I set 63 to 65% and the jet would naturally hold the approach speed. When I flew C-141Bs, we used fuel flow. We use fuel flow in the Gulfstreams I fly also. The only time I would keep autothrottles on with autopilot off was doing circles. The DC10 had a CWS mode between on and off with the AP lever. During the circle, the plane would hold what ever bank I input and allowed me to keep altitude hold. This way I could be outside more and not worry much about altitude and speed. Once ready for descent, click it all off and maneuver for landing. I know some companies were directing autothrottle use due to fuel savings. In this case, guys/gals were leaving it all on til a point at which they can take it all off. I do not like landing with autothrottles on while manually flying because it targets a hard AGL and rate of decrease. When flying heavies with a wide landing range, the flare and power pull is different at different weights. In the KC/DC10, it started at 50ft and had a slow rate. If you were heavy, it's fine, but leads to floating since the power is slowing coming off. The jet is heavy and has alot of momentum, you need to get the power off to prevent floating. If the jet is light, it responds quickly to power changes. This leads to firmer touch downs. Both of these causes issues and makes you flare differently from standard.

    In the G5/G550, it starts at 50ft and has a slow pull. Problem here is that you get excessive floating. In the G5, throttles need to be at idle at 50ft. It's very slick and loses the plus 5kts slowly. In the G550, you have to be at idle at 100ft. Landing with autothrottles on will completely ruin your landing. Do it on a short runway and you will have a career altering event.


    The 717 actually recommends leaving autothrottles on even when flying manually. Rear mounted engines don't throw of the pitch as much. Here is an exert from our BBJ(737) manual.



    Autothrottle use is recommended during takeoff and climb in either automatic or


    manual flight. During all other phases of flight, autothrottle use is recommended


    only when the autopilot is engaged in CMD.

    Not being a pilot, I'm curious which is the "real" way.

    In the real world it's aircraft dependent. Traditionally, if auto pilot is engaged(auto land capable), the throttles are engaged through touch down. If Auto pilot is off, the throttles are disengaged. This is mainly Boeing types due to the pitching moments produced when thrust is changed. When I flew DC-10-30s, we didn't have autoland on our jets, but was allowed to land with throttles engaged. They would just retard to idle at 50ft. Though you could do that, most guys turned both off. In the GV and G550 that I currently fly, you can leave them engaged also through landing when manually flying, but no one does it. They just follow the traditional way of all on or all off. Most guys will kick off autopilot and throttles at 1000ft clear weather or 100ft to 200ft with a low ceiling. If the plane is not certified for autoland, you should be taking it off at the aircraft's min altitude for auto pilot. 300ft to 1000ft is a good place to kick them off since it gives you a chance to warm your hands up and get a feel before landing.

    I've noticed that the flaps on the Q400 and the jet aircraft seems to be low. As you add flaps/gear, there should be a need for increased power. It takes some time to slow with flaps extended and throttles idle.

    Once they move to real world nav database, you will be able to plan a flight using real world departure procedures. The way points from the real world are missing. When departing and arriving mountainous terrain in aerofly, you have to be visual. Once they use a nav data base, you can use departure routes and only worry about being at way points at minimum altitudes.