Flight Simulators and Airlines

Did you know that for many pilots, the first time they fly a particular large aircraft, there are paying passengers in the back? Scarey, not really, since those pilots probably have hundreds of hours in simulators. These simulators are big expensive devices, that operate very similar to the real aircraft, and give the pilot almost all the sensations of being in an aircraft.

 The simulators that the airlines use have rules similar to aircraft. 14 CFR part 121 appendix H covers Advanced Simulators. These simulators are typically held in a specially built building. The simulators weigh several tons, and are up on legs (usually 6) that move with hydraulic or electric actuators to give the occupants the feeling of being in flight (or landing). The legs are usually isolated from the rest of the building, both for extra support and to prevent the other parts of the building from feeling the motion.

There are several computers that run these simulators. Some of the computers make the instruments behave like the aircraft in flight, others generate the scenery that the pilots see out the "windows" and other computers generate the motion that give the occupants the feeling that match the instruments and the scenery. These computers are generally quite powerful, to minimize any lag between the pilots inputs and the changes visible and felt. It is especially important to have the motion match the visual, to prevent the occupants from getting any form of motion sickness.

Like anything mechanical, the simulators will break on occasion. The extent of the break will cause the simulator technicians to determine if the simulator meets the FAA requirements. If it can be used, it may have a minimum equipment list deviation (MEL). If the simulator is broken too bad, it may not meet FAA requirements, and will be taken off line until repairs can be made. Any MEL deviation can have a list of operations that cannot be performed, just like a transport aircraft.

Changes to hardware and software are covered by the same FAA 14 CFR 121 appendix H. Basically, what the simulator was built with, is what it must have every day. Occasionally the manufacturer will make major modifications to a particular simulator hardware, but that will be rare. Many simulators are over 10 years old, still running the old hardware from when they were new. Technicians must keep 10 or 15 year old hardware running, and source spares for them. Most simulator manufacturers will choose OEMs who understand that and make every effort to insure the OEMs will have enough spares to keep these expensive devices operating 24x7x365.

The software will get changes on occasion. This is normal, and is less expensive than replacing hardware. The FAA requires review of software for any impact to the performance of the simulator. The software review request must be sent to the FAA at least 21 days before using on the simulator used for training. The software must be test flown, and checked for any performace changes. If the aircraft performance changes (IE due to SB or AD), then it will be mandatory for the simulator to be upgraded to match the change.

As life goes, the FAA also makes changes. Navaids change, runways change, and other facilities change (new terminals, control towers, etc). Most simulator operators have staff of technicians that will keep the simulator data up to date. Every 28 days the FAA publishes new information (AIRAC cycle). The technicians must keep the simulators navaids and routes matching the current charts. Over 10 or 15 years, scenery at airports change, along with many scenery items. The technicians must keep all this information current as well, so the pilots know what to expect when they get to a particular airport.

Operating these simulators can be very expensive. Some estimate that a full motion simulator for a part 121 aircraft can be over $1000/hr. When the technicians time, as well as building costs are factored in, it may be more costly to operate the simulator.

For a new pilot to come into the full motion simulator, they would spend most of their time just getting familiar with the basics. Throttle quadrants and yokes are pretty standard, but it is all the other systems that the new pilot needs to know, and take so long to find. There are other simulators for getting familiar with the basic systems.

The most basic simulator, is the cardboard cockpit, also known as a procedure trainer. This procedure trainer is usually in an office, with some tables covered with cardboard printed with switches and indicators that look like a cockpit. The pilots will use this to set up the "flows".  This allows the pilot to go through each checklist, and know where the switches are and what indicators to look at to verify a particular operation. The pilots can also use the procedure trainer to go through emergency and other off normal operations.

The next simulator will still be called a procedure trainer, but will have operating switches, yoke, throttles and displays, but probably no scenery out the windows. The advanced procedure trainer can be used for flow checks, and also instrument procedures. Many of the same computers are used in the advanced procedure trainer, but will not be as complex, since it is missing the scenery and motion.

The full motion simulator, and the advanced procedure trainer have an extra computer. The instructor console is outside of the cockpit area, and is used to set up various training scenarios. Scenarios can altitude and position settings, allowing pilots to train on specific aspects of a flight. Other settings can include weather, and mechanical issues. 

All simulators require maintenance. There are various people involved in the simulator business. Pilots are well trained before ever stepping foot in the cockpit of a real aircraft.

How To Design an Autopilot

An autopilot is a very complex bit of machinery. Ever think about all the stuff it needs to know and how it does it so well? As a pilot, you were taught that you start a turn before you get to a waypoint, so you don't overshoot. As a computer, how do you know to start a turn before, and how much before?

The FAA has an amazing document, AC 8260.58. I recommend getting the whole PDF, and opening it in a true Adobe reader because this document is interactive, and it is large. This document is full of very well explained math. There are tons of definitions and discussions in here, PLUS there are sample calculators right in the document.Depending on the age of your Adobe reader, you might get some weird document navigation indications.

The document is really about how PBN procedures are designed. PBN procedures assume the aircraft will have an autopilot, so it is interesting to see how the procedures are designed for the limitations of the autopilot.

The drawings in the document are amazing. Look in volume 5 page 9 figures 2-4A and 2-4B, to see how turn errors can be calculated using the radius of the required navigation performance, and bisector lines, and the arc that intersects the edges. The page before (page 4) explains step by step how to make the calculation.

The document is full of acronyms, but they are all explained in volume1 chapter 3. Not all the acronyms are what you might think or are used to in other contexts. In this document CG is "Climb Gradient" not "center of gravity", ATT is "Along Track Tolerance" not that big phone company, etc. It is best to have that chapter book marked to allow you to check back.

The book outlines other tolerances as well. Some of the measurements are metric, while others are nautical miles, and the conversions are part of the calculators in the document. Many of the intermediate values need to be kept in 15 significant digits, and stored in 64bits, with no rounding of intermediate results. There are other common standards that this document relies on, including GPS units calculate the diameter and shape of the earth based on WGS-84 standard.

Much of what is in the document is code that could be copied right into some programs. What code isn't there, is mostly easy to figure out, and could be part of a separate function or method. 

There is a good review of many basic algorithms, including intersections of two arcs, locating a point relative to a locus, and calculating arc length, or sub-arc lengths. 8260.58 might be a good workbook for some advanced STEM type program.

With this reference, building an autopilot should be less searching, and easy implementing.

Resources

Wow, the internet is full of documentation. If you go back to the roots of the world wide web, people like Tim Berners-Lee and before him even Ted Nelson with project Xanadu (and go back to the Manhattan project with Vannever Bush it isn't a new idea at all) the idea was to share information. Learning material, instructions and books could all be connected.

This blog is all about sharing and linking. This is information I have gathered either professionally, or just as part of learning something else. Certainly as a pilot, I have had to learn some of this, but the theory came probably years before as a kid getting my ham radio license, or building my first robot. I sometimes paraphrase other documents, and even copy stuff here from other places.

The greatest source of verified information I share is from wikipedia. I may know something about something, but I usually try to verify it with some other source, and searching Google will usually point me at an article in wikipedia.

The math help I get from the aviation formulary web site. I don't know Ed Williams, I'd love to meet him. I have used his site for years and years writing aviation software. This is well presented, and detailed or not as necessary. This site focuses on the math, and that is all. Good stuff!

For charts and routes, I tend to rely on SkyVector.com. I can't believe this resource is free, and I appreciate it. Flight aware has plates and ASDI data, so it is also useful, similar but different. For airport information, I use Airnav, which has links to other sites including vfrmap.com and FAA's pilotweb.

Weather needs are met by the NOAA ADDS web site. There is so much information here that the weather channel, weather bug and other weather sources make really hard to look at. If you want to really forecast what will happen on your route of flight, ADDS is the only source.

Buying and selling of aircraft and parts, I rely on Barnstormers.com. The baroness runs a great web site, and I get the weekly email. I can't not mention Trade a plane, since if I were to sell my plane, I would list it on both sites. I still like the physical paper of trade a plane, but right now I am not subscribed. I also shop ebay for some aviation items also. For building supplies Wicks Aircraft, and Aircraft Spruce are the main sources.

There are several resources for electronics concepts. For builders Bob Nuckolls Aero Electric Connection is the main place. If you want to build your own avionics, there is a page dedicated to homebuilt avionics.

These are my go to pages, what are yours?

Collaborative Descision Making or Why Is My Flight Late

The FAA and the airlines (and some business aviation) have an agreement. If there is no space for the airplane, leave it on the ground. The FAA has a set of systems monitoring all flight plans, scheduled flights and the weather, mushes it all together to determine if there is room for the aircraft in the National Airspace System. If there is too much traffic at a certain point, then the FAA will issue a program, and tell the airlines to keep their aircraft on the ground.

If there is fog at an airport, and the rate the airport can take aircraft in is reduced to less than the volume of scheduled flights into the airport, then the FAA will issue a ground delay program (GDP). This ground delay will cause flights to be delayed at the departure airport. Typically the delay will  begin after the weather starts slowing things down, and will continue until the forecast shows things will get better.

Most passengers find this frustrating, but the reason is quite sound. The flight is going to be late to that airport anyway. Rather than risk stranding passengers at another destination, because the aircraft had to hold waiting for a slot at the destination, and the aircraft ran low on fuel, the passengers are left at the departure airport, and can choose a different itinerary, or just wait it out.

Other programs include an airspace flow program (AFP). An AFP is used when there is a line of weather across several states, causing many flights to be planned to go around that weather, all at a single choke point. The choke point is like an airport, where not all flights can get through, and may have to hold to allow proper spacing. Rather than getting too many aircraft through a small hole, it is better to leave the aircraft at the departure airport, with a delay, rather than possibly holding, and diverting.

Any of the delay programs are just a delay. The flight is delayed a known amount based on forecast weather. Sometimes, the delays can be adjusted based on current local forecasts. Sometimes the weather is better than forecast, other times not.

The last major program is a full Ground Stop (GS) program. This is where the conditions at the destination airport are not conducive to landing any more aircraft. One scenario for a ground stop would be malfunctioning equipment at a destination, and the time to repair is not determined.  The delayed aircraft may get an updated expected departure time, or they will get a next update time.

With the DOT 3hour rules and other reasons, some airlines may elect to cancel a flight or two due to one of these programs. Cancelling flights allow space to be made available. The FAA allows airlines to trade these spaces, or hold them for other flights. If airline A decides that it would be better to delay flight 123 that only is 60% full, they may decide to put most of those passengers on flight 456 that is 50% full, but leaving an hour later. That way the airline can make 48 people only a little late, and they will tell their friends how the airline "saved" them. The other 32 will just be frustrated, and may not complain too loudly.

The swaps and adjustments are part of the Collaborative Decision Making (CDM) program the FAA organizes. The CDM organization actually has several arms, and is chaired both by the airlines and the FAA. Some of the arms include groups working on weather, ground movement, and flight planning.

One of the programs the CDM group is working on, is collaborative flight planning. If the FAA and the airlines could make plans based on know congestion areas, then some of the programs might be eliminated. If the FAA gets a bunch of flight plans that all show the flights going around the south end of a storm, the FAA can suggest that going the north way might get the flight there sooner, even though the mileage is longer.

There is still a long way to go with all the information the airlines and the FAA  have to get passengers to their destination when they want to be there. Things are getting better all the time.

How Can Flight Planning be Accurate?

In pilot training, we all learned to use an E6-B or something similar, along with winds aloft forecast to do our flight planning. The values were pretty good, about as good as we could get considering it was all we knew. If the trip was short (about 500 miles or less) being careful, it can be nailed pretty well.

Now imagine flying a jet, over 2500 miles non stop, flying airways, in or near the jetstream, with an E6-B and the winds aloft forecast. It can be very accurate, but will be very time consuming to calculate. The interpolation alone will be a tremendous effort, and then calculating along the route, and keeping it all added correctly can be quite a challenge.

The computer has a small advantage in calculating the flight plan. Computers are great at keeping track of all the numbers, and interpolating. The National Weather Service (US NWS) and UK Meterological Office (UKMO) all model the weather in little rectangular 3D shapes, that the computer can look at and determine winds and temperature for the airspace.

There are multiple wind models used in various systems for flight planning. The Global Forecast System (GFS) model is pretty good, and available almost world wide. The GFS model is run 4 times a day, and forecasts can go as long as 16 days, with the first day having a specific forecast every hour, starting the second day every third hour up to 8 days, and every twelfth hour up to 16 days. The 3D rectangles are about 27km square and about 500 feet thick. The US also produces a finer grained model, called RAP or Rapid Refresh model which is updated more often has smaller 3D rectangles (13km squares, by about 600feet) and only covers the CONUS for 12 hours. (A high resolution rapid refresh model (HRRR) is being developed with 3km squares).

If a flight plan is created that will take an aircraft from Los Angeles to New York, flying at FL370 using the route:

  OSHNN4 DAG J100 LAS J146 DVC PUB SLN SPI J80 CREEP APE CTW KODIE PSB LVZ LENDY6

The computer will look at all the 3D rectangles that the aircraft will pass through, and using the winds and temperature for that airspace, calculate the ground speed, distance and time for the aircraft in that area. The accumulation of all the times will be accumulated, and the end result will be the total flight time. That result is most of the time, then the book values for the aircraft can be used for climb to, and descend from altitude.

Normally the flight planning computer will expand out the OSHNN4 departure to all the waypoints, and calculate them individually to top of climb (TOC) then start calculating in the in the 3D rectangles at altitude. The flight planning computer will expand out the LENDY6 arrival and determine the top of descent (TOD) calculating everything up to that point using the flight level 3D rectangles, and the proper rectangles on the way down.

For airlines, a dispatcher will normally generate these flight plans. The dispatcher will look at the flights coming up for their area, and maybe run a preliminary plan (just an FYI, the Jeppesen Jetplan Flight Planning Engine takes about 6 seconds to run a plan similar to the Los Angeles to New York above). The dispatcher can look at the flight plan result, and plot it on a map to see where there may be weather or other congestion, and adjust it accordingly.

Once the dispatcher is happy with the plan, they will file it with the FAA ATC. The FAA ATC can look at that plan, and suggest changes or let the pilot fly it as it is. The FAA will keep the plan on file and use it in various calculations, including URET and other collaborative decision making (CDM) systems.

Overall, the computer makes flight planning much easier. There is much more to flight planning, this only covers winds and temperatures aloft, and how it applies to flight planning.

What should I cover next?
   

trying to get ahead of the learning curve

Over the last couple months, I have been behind the learning curve in a big way. I started a new job in a new domain, and I decided to get my instructors rating. Since I haven't flown for a couple years, I decided to concentrate on my ground instructor rating, then I can go look at flight instructor another time. Two big domains of information cramming stuff into my little brain has been a challenge finding time to explain stuff. (nevermind the holidays and family issues, and all).

The FAA is changing the testing for everyone, and that could be a good thing, but it may not be. As part of the advanced ground instruction (AGI) rating I completed recently, there were two distinct parts. There was the flying instruction, and the fundamentals of instruction (FOI) part. The flying part was relatively easy, since I feel pretty confident in my understanding of the fundamentals of flight. The ground instruction test was pretty easy, and the questions and answers mostly matched the test prep I used.

The FOI was a real challenge. The FOI test is the one the FAA has changed significantly in the last couple of years. The main book for the test is the Aviation Instructors Handbook (FAA-H-8083-9A). It looks a little more colorful than most of the other FAA documents, and seems to have some really good information in it. Lots of learning theory, and psychology presented in a way to help the reader understand how people in general, and pilots in particular learn. Things that may offer help in overcoming difficulty when instructing different individuals.

The FOI test took a left turn from there. There are plenty of really informative good ideas in the book, but the test rather than focusing on those items, decided to test on nuances that may not be applicable. One example of the test, it asked the details of Bloom's Taxonomy of the Cognitive Domain, and not why they matter, or how to apply them. Like in a flight while teaching, I will consider Knowledge, Comprehension, Application, Analysis, Synthesis, and Evaluation, when  dealing with a student about to cross control an aircraft in a base to final turn. As we are spinning into the ground, I will think more about the application of control inputs, and how I should have emphasized them more, without considering the students comprehension.

Other places in the FOI test, they put in three perfectly good answers. I know at least twice in the test, I said to myself, "all of the above". Other places, there were three answers that contradicted the book. A question about the PTS asked what they are for. They are for testing, but the answer choices where all over the place, were they teaching aids (well the book says introduce them in the last three hours, sounds like a teaching aid to me).

I felt like I worked really hard preparing for this test. None of my practice tests since Christmas (when I re-read the book) did I get less than an 82. When I got done with the real test, I knew I didn't do that well and felt my score, if I passed, was just above 70. It was above 70, but not by much.

I guess end of the day, in 3 years when teaching a class no one will ask me if I got a 99 on the test or a 71, I passed, and I will continue to learn, and part of being a professional includes research (that was another question on the test). I shall be a professional.

What will I do with my new knowledge, and skill? I hope to introduce people to the concepts of flying. I want my classes to be broad enough that it will answer peoples questions about how aircraft work, with enough detail to allow the students to pass the private pilot ground school at the end, and give them the tools to do something with their knowledge. I am considering another blog around teaching people to fly.

What do you all want to know?

Air Data and FMS

I missed a bit in my last post about FMS's. (I am shortening everything to FMS,
it is mostly the same thing, a system). Air data, what is that...

Air data is usually known as the pitot static system on smaller aircraft. Getting the pressure information into the aircraft involves a analog to digital (A to D) conversion.

All aircraft have pitot tubes to measure air pressure because of speed. Pitot tubes are closed tubes, and the airspeed indicator, or air data computer only can measure the pressure of the air trying to get into them. Larger aircraft may have multiple pitot tubes on them, for redundancy mostly. The airspeed indication is made by comparing the difference in pressure from the ambient air, and the pressure forced in the pitot tube.

The ambient air pressure is entered to the system through static ports. There are multiple static ports on most aircraft. The static ports also help measure altitude and rate of altitude change (vertical speed indication). Again the static system measures change in pressure, from ground level to altitude.

There is a relativly simple formula (thanks to wikipedia) for incompressible fluids  pt is total pressure, ps is static pressure, p is density. V becomes the fluid velocity, or airspeed for us.




Air is easily compressible, and that makes the formula a little more complicated, since you have to integrate pressure and stagnation values.

Static pressure is a little easier. The air pressure doesn't change at a constant rate as the aircraft flies at a higher altitude, but the curve is relatively constant. Thanks to engineering toolbox we can use:

  p = 101325 (1 - 2.25577 10^-5 h)^5.25588  

p is air pressure (millibars) and h is height above a fixed point in meters. To get the whole formula, you need to include temperature, and humidity as well, see the wikipedia entry if absolute MSL needs to be measured.   

A small transducer can measure the different pressures, and provide a voltage that the computer can read. Computers are good at math, even complex math, allowing us to have usable information on heads up displays, tapes and other graphical presentations.

The air data computer reads these transducers, and puts the data into a usable format that the FMS can use.

The FMS allows setting marks called bugs on the instruments. If the pilot wants to fly at 250kt indicated airspeed, they may enter a command on the CDU keyboard, it will display on the airspeed indicator. The FMS also allows setting heading bugs, and feeds the flight director when flying on a flight plan.

There are more things in the FMS as well, and as I have time, I'll keep adding to the blog.

Write, and let me know what you think