Hi fellow wiki editors!

To help newly registered users get more familiar with the wiki (and maybe older users too) there is now a {{Welcome to the wiki}} template. Have a look at it and feel free to add it to new users discussion pages (and perhaps your own).

I have tried to keep the template short, but meaningful. /Johan G

A-4F Skyhawk Operations Manual

From FlightGear wiki
Jump to: navigation, search

1rightarrow.png See Douglas A4 Skyhawk for the main article about this subject.

By Andy Ross (2006)

Fundamentals

Grains of Salt

I am not a real-life pilot, and I am certainly not a Naval Aviator. The information and instructions here are based on my own research and experience with the FlightGear A-4 flight model. Most of it has at least a basis in truth, but all of it contains at least a little guesswork on my part. Anyone with better sources should feel free to correct this document as needed.

On the whole, I find the FlightGear Skyhawk to be challenging and realistic to fly. Unlike most (perhaps all) consumer military simulators, the flight model for the A-4 is not "dumbed down" to make dogfighting easier for novices. All of the same caveats and procedures that I have read about in books seem to apply well to the FlightGear model. Most other simulators seem to have one or two areas which just "feel" wrong and/or fail to match presumably authoritative sources.

Flight Qualities

The Skyhawk has a low aspect ratio delta wing. This means that it experiences a very gradual increase in lift with angle of attack, and only a very mild stall at a much higher AoA than a straight-winged aircraft would. While this makes handling generally more benign, it has two important effects that a pilot transitioning from lightplanes must be aware of.

First, while there is no sudden loss of lift at moderate to high AoAs (20-30 units) in the Skyhawk, the induced drag continues to increase with AoA. This means that, when producing maximum lift, the A-4's wing is also producing far more drag on a relative basis than the conventional wing would. The net effect is that low-aspect wings "bleed energy" when at high angles of attack in ways that can be surprising to straight-wing pilots who tend to assume constant speed across maneuvers. This applies both to high-G turns and also to approaches, which are flown in the same high-AoA regime. While a Cessna 172 can change attitude and climb for a go-around with a simple throttle setting, the A-4 is capable of only a very modest climb in approach configuration.

Second, the ability to "use" those higher angle of attacks means that the pilot's stick is correspondingly more sensitive. Full back-stick at stall speed in a Cessna will produce perhaps 16 degrees of AoA. No more elevator authority is needed, because beyond that the aircraft will be stalling anyway. Full back-stick in the A-4 produces at least twice as much pitch change. The pilot needs to be aware of how much elevator is needed for a given maneuver. Simply pulling the stick back to the stop is likely to be very surprising and lead to (depending on the flight environment) a stall, an airframe overstress, or a departure and/or spin. Be gentle.

The A-4's ailerons are tremendously effective. At 350 knots, the real aircraft has been measured to roll at 400° per second, which is matched rather closely by FlightGear. This can be loads of fun, but it also makes "rolling into" a tight turn rather touchy. Be prepared to use some reverse aileron to halt your roll at your desired bank angle or else you may well overshoot.

While the A-4 is an attack jet, and therefore much zippier than a 747 or Cessna 172P, its thrust to weight ratio is not in the "why bother with wings" class such as modern fighters like the Su-27 or Rafale. You cannot simply point the Skyhawk's nose to the sky and expect it to climb, nor can you escape from piloting errors simply by applying throttle. It is a pilot's aircraft, and needs to be flown within its capabilities.


Cockpit

A4-F-Panel.jpg

The Skyhawk cockpit is simple and spare, even when compared to its contemporary aircraft. Unlike modern fighters, all the gauges are convention analog ones and there is no HUD. Nonetheless, the cockpit should be accessible to anyone with experience in lightplanes such as the Cessna 172. The airspeed indicator, vertical speed gauge, and altimeter all operate identically to the ones you are familiar with, and there is a conventional HSI/gyrocompass at the bottom of the panel.

The "scrolling card" artificial horizon is replaced in the A-4 by a full three degree of freedom attitude indicator which shows heading in addition to pitch and roll. It has no travel or rotation limits, making it suitable for aerobatic flight. At the bottom of the instrument (hard to notice if you do not know it is there) is a turn rate indicator.

Something not seen in passenger aircraft cockpits is an accelerometer in the top left corner of the panel. This measures vertical pilot acceleration in G's, and is useful in maneuvering flight to tell the simulator pilot what the real pilot would be feeling. It is also useful in IMC conditions to detect trim/airspeed mismatches. An aircraft flying at trim speed will have show exactly 1G on the gauge. The accelerometer can thus be used as an aid to trimming in conditions where there is no visible (and pitching) horizon to look at. It has a higher resolution for this purpose than the attitude indicator.

The engine in the A-4F is a Pratt & Whitney J52-P8A non-afterburning turbojet which develops 9300 pounds of static thrust under standard sea level conditions. Thrust on a jet is typically measured using N1 (first stage) turbine RPM, expressed as a percentage of the maximum continuous operating speed. (Actually, the real A-4 has an exhaust pressure ratio gauge as well, but that is not modeled yet). The engine is allowed to run as high as 105% during takeoff, but should be kept under 100% at cruise. Be aware that jet engines, unlike piston engines, take several seconds to "spool" to a new power setting once the throttle is moved.

Angle of Attack

In Navy jets, approaches are flown at a specified angle of attack rather than at particular airspeeds. This allows for a single set of procedures to be used for landings at all gross weight conditions. In a commercial jet flying a predetermined flight plan, there is plenty of time to look up values for approach speed based on current fuel load. When landing an attack jet on a carrier, that is less of an option.

There are two angle of attack indicators in the cockpit. The first is a rotary dial at the top of your panel, which indicates angle of attack in "units". These are not degrees, but simply arbitrary numbers specific to an aircraft model. There is a small tick mark at the 3 o'clock position that indicates the ideal approach AoA. Final approaches under all conditions should be flown at this AoA.

To aid in comprehension during approach, there is also an AoA "indexer" at eye level attached to the windscreen. This is a 3-element lighted display which displays five conditions:

  • on speed (yellow center circle)
  • slightly slow (yellow circle and downward-pointing arrow indicating "push the nose down")
  • slightly fast (yellow circle and up chevron indicating "pitch up")
  • very slow (no circle, green downward chevron meaning "pitch down and speed up!") and
  • very fast (no circle, red upward chevron meaning "pitch up and slow down!")

Takeoff

  1. Flaps up. The A-4 uses split flaps, which create too much drag to be useful as takeoff lift enhancers. The wing slats are automatically actuated, and do not require pilot operation.
  2. Elevator trim neutral, or very slightly nose-up. Do not trim for approach speed, as this is too slow for a takeoff climb and will cause the nose wheel to lift too early in the takeoff run.
  3. Brakes off. Throttle to full. Begin takeoff roll.
  4. Rotate. Rotation speed for full fuel and no external stores is 145 knots. Rotation in the A-4 is a rapid process, pull the stick back to approximately one half travel, raise the nose to 20-25 units of AoA and hold it there. The aircraft will lift off into ground effect.
  5. Gear up immediately.
  6. Do not attempt to climb yet. Best climb speeds in the Skyhawk are 300 knots or greater. Simply hold the nose steady against the horizon and allow airspeed and vertical velocity to increase gradually. Alternatively, on long runways a "Blue Angels" takeoff has the pilot (carefully!) hold the aircraft in ground effect as AoA drops and speed increases. Then, at 250-300 knots, pull up into a 2-3G climb to a deck angle of 45° or so. This is lots of fun and looks very cool in external view.

Landing

Approach and Pattern

Here(Wikipedia) you find a flight chart

  1. Plan on arriving over the airfield at 1000-2000 feet AGL and 350 knots. At cruise speeds, the A-4 is a very slippery aircraft and takes a long time to slow down, so be prepared.
  2. Overfly the desired runway while descending to 1000 feet AGL. Note the runway heading on your gyrocompass. The A-4 has poor rearward visibility; you will not be able to see the runway for reference during significant portions of the landing pattern, and knowing you are on the correct heading is important.
  3. At the end of the runway, cut throttle (if you have not already) and pull into a 2G, 180° turn to the left. This is called the "brake", and is intended to reduce speed.
  4. As speed drops through 250 knots, lower the gear. At 200 knots (you should now be in level flight on the downwind leg) begin adding flaps gradually as the AoA increases. The goal is to get the aircraft trimmed (and powered) for level flight at the on-speed AoA of 17.5 units by the end of the downwind leg. Warning: with full flaps and at on-speed AoA, the aircraft will want to descend very steeply. Increase throttle proactively as you add flaps, or you will pancake.
  5. At the end of the downwind leg, approximately one mile from the runway threshold, begin the turn onto final. In approach configuration, the aircraft must be handled gently; use no more than 30° of bank. Watch for the runway and adjust as necessary. The goal is to roll out of the turn on glideslope and on speed.

Actually, the simplest approach procedure has the pilot line the aircraft up on the centerline far out (7+ miles) from the threshold and at 3000-4000 feet AGL. Pull a hard turn onto final to brake, drop the gear and the flaps, trim to on-speed, and only then start worrying about lineup and glideslope. You will be very high, but the aircraft descends very steeply in approach configuration. Simply drop to the proper glideslope, apply power, and then nurse the throttle all the way to touchdown.

This provides more time per approach to practice the very important throttle technique without having to worry about flying a level pattern at 1000 feet AGL while in approach configuration (which is also very difficult). You can then practice flying level turns in approach configuration, and put it all together later.

Final Approach

A4-F-Approach.jpg

Fly the final approach using power to control pitch and altitude. This advice is repeated in pilot manuals everywhere, but nowhere is it more important than when flying a jet at very high angles of attack. If you try to correct for a low approach by pulling back on the stick, you will create only a little bit more lift and a lot more drag. Your airspeed will drop rapidly and you will pancake into the ground well short of the threshold. To make matters worse, the engine takes several seconds to "spool" to a new power setting once you move the throttle, making this kind of pancake maneuver very difficult and dangerous to recover from once it happens. Successful landings involve careful attention and practice. Hints and tips that I have found useful:

Trim well. If you fly the approach while trimmed for the proper AoA, then the aircraft will always be on-speed, or very nearly so. If you try to fly a constant AoA approach with the stick, you will rapidly find yourself becoming insane (or dead). Once trimmed, you should need nothing more than throttle and gentle aileron control (and perhaps a gentle nose-down push on the stick once in a while to correct a small glideslope deviation) throughout out the approach.

Get your eyes out of the cockpit. The gauges do not tell you enough. Instead, watch the position of runway threshold. With the aircraft trimmed at approach AoA (you do have it trimmed at approach AoA, right?), the aiming point should be placed about 1/3 of the way from the top of the panel to the windscreen crossbar. If you are too high, you should be aiming the threshold a bit higher on the windscreen; too low means you should be trying to lower it.

Remember how power and trim interact. The trim (again, you have trimmed the aircraft to fly on-speed, right?) selects an airspeed for level flight. If the aircraft is flying faster than trim speed, it will be producing more lift and (this is the important part) pitching upward. If it is flying slower, its nose will be dropping. Pay attention to this tendency. If you have applied power to reduce your rate of decent, but the nose is still going up, reduce power to keep the aircraft at on-speed or else you will overshoot and climb too high. If the nose is dropping nicely to where you want it, start adding power to reduce the rate of drop before you reach the attitude you want lest you drop below the glideslope.

Don't look at the engine RPM gauge once you are stable on the glideslope (unless you think there is a problem, of course). Approach power settings will vary widely depending on aircraft gross weight. Learn to use small throttle movements and listen for the changes, rather than trying to find a specific RPM setting. Try flying approaches at many weight settings to drive this point home; if you are using the RPM gauge as a crutch, flying at different weights is a good way to break this habit.

Bug somebody to implement approach lighting. On real runways, you will have a VASI or PAPI to tell you where you are on the glideslope. On carriers or at Naval Air Stations, you will have a "meatball", which is broadly similar but provides more information - see the carrier "Nimitz" in FlightGear. In the meantime, you can use your ILS receiver as a proxy. With practice, however, I have found that making an approach without glideslope information is not excessively difficult.

Touchdown

Do not flare. To begin with, flaring is not part of Navy procedures. Aircraft landing on carriers do not have that option. It is also not easy to do at high angles of attack. A Cessna will cross the threshold with lots of "reserve lift" available to a pilot who pulls back on the yoke, but the Skyhawk is much nearer to stall AoA and producing far more drag than the lightplane would be. It is very easy to "flare" into an unrecoverable stall 30-40 feet in the air.

Instead, simply fly the aircraft onto the ground. If you are on the correct glide slope, your vertical velocity should be 700-1000 feet per minute. At this speed, the aircraft should land firmly but solidly on its gear. If you are coming in shallow, it is possible that it will settle into ground effect without actually touching down. If you are coming down fast and do not have time to correct the glide slope with power, be prepared for a bounce (but again: do not try to correct this with the stick).

Do not cut power until you are either on the ground or level and in ground effect. It is possible, and likely if you are coming in steep, that the aircraft will bounce on landing. If you cut power before the bounce, you will suddenly find yourself 20-30 feet in the air with an idle engine and well below stall speed. This is bad.

As the aircraft touches down, immediately apply some forward stick to keep the main wheels on the ground. Retract the flaps. Slowly push the nose wheel onto the ground with more forward stick. When trimmed for approach, the aircraft will want to bob up; keep the nose wheel down. Once all three wheels are down, brake.

Alternatively, if you have a long runway, leave the nose wheel in the air and slowly apply back stick until it settles down on its own at 80-100 knots before braking. This minimizes use of the brakes by allowing aerodynamic drag to slow the aircraft as much as possible, and works well also. I don't know what standard Navy procedure was for the landing rollout.

1rightarrow.png See Douglas A4 Skyhawk for the main article about this subject.