GPWS, TAWS and HTAWS

Controlled Flight Into Terrain (CFIT), and other troubles are common for all aircraft. Obstacles are out there, the trick is avoiding them. The FAA has come up with various equipment that can help minimize the surprises obstacles may offer.

beware...lest the ground rise up and smite thee

It wouldn't be an FAA requirement if there wasn't a technical standard order (TSO) to go with it. TSO-C151B covers terrain awarness and warning systems (TAWS). Ground Proximity Warning Systems (GPWS) are covered in this as well. Helicopter Terrain Awareness and Warning Systems (HTAWS) are covered in their own TSO-C194. For the most part, all the systems work similarly, the main difference is the performance of the aircraft.

Most TAWS systems work by knowing where the aircraft is and checking a terrain database to know if there are obstacles in the path of the aircraft. Knowing the trends (is the aircraft climbing or turning, etc) will allow a computer to know how much of the database needs checking. A current altitude source is needed. Sometimes this could be the barometric altitude, other time it may be a radar altitude.

There are various databases of terrain available. Either from the FAA or other sources. The FAA makes available the current database, and a daily update that will include construction cranes, and other changes to obstacles.  The daily update file (DDOF) is available as the whole current database, and also just the changes.

The TAWS must be looking along the path of flight to function. The distance ahead is critical, especially at cruise altitude and speed. While most 121 aircraft are capable of clearing all but the highest mountains in the world, the assigned flight level or performance with fuel and passengers may not allow the current flight to climb over all the enroute terrain.

During approach to landing, and departure, there can be many more obstacles. Buildings, trees, construction cranes, and radio towers can be near airports. The TAWS must also consider these obstacles locations and provide safe clearance to avoid them.

Generally the TAWS should provide warnings if the aircraft is or will be between 100ft and 400ft of an obstacle during departure, and less than 1000ft during enroute portion of flight.

Within the TAWS there are various classes of capabilities provided by the systems. The classes are broken down as:

• Class A: Alerting based on; excessive rates of descent, excessive closure rate to terrain, negative climb rate after take-off, flight into terrain when not in landing configuration and excessive downward deviation on ILS approach. It will have voice alerting and sweep tones. It must also have a terrain display. 
• Class B: Similar alerts, but with wider tolerances as the class A alerts. Class B assumes no Radar Altimeter, and the base threshold of the runway altitude. There must be an altitude call out for 500ft and other alerts, but class B TAWS does not require a display. 

TAWS systems are required on some aircraft. All part 121 turbine aircraft must have a class A terrain awareness and warning system (121.354). All part 135 turbine aircraft that seat 6-9 passengers must have a class B TAWS (135.154). Even part 91 turbine aircraft that seat more than 6 seats must have a class B TAWS. More details are in AC25-23. 

While TAWS have not eliminated all CFIT accidents, there are many situations where the systems have proven to save lives. 

The FAA is Still Trying To Save Face

ADS-B again...

The FAA is offering relief from the 2020 deadline, only this time is not just Part 121 air carriers but all aircraft, even GA!

It is all pretty qualified, you still need to try to meet the 2020 deadline. The only relief is really in the position source and it's level of service. For ADS-B the FAA wants C-145/146 WAAS type level of accuracy. That is a good thing, since the more accurate your ADS-B location is the fewer alerts others may get.

The is this old joke:

Tower: "Alpha Charlie, climb to 4000 ft for noise abatement" 
Pilot: "How can I possibly be creating excess noise at 2000 ft?" 
Tower: "At 4000 ft you will miss the twin coming at you at 2000 ft, and that is bound to avoid one hell of a racket".

And with poor location indication, something like this could happen.

Will the FAA be official about any of this anytime soon? Probably not. They most likely don't want you to know anything about it, and would rather have all aircraft fully compliant on 2020. But for folks with older GPS receivers or pre-2013 ADS-B capable installs it will be nice to not have to rush for a full panel upgrade.

It only relief. It isn't full abandonment of the rules. The full rules will still go into effect 2025 or so they say.

I know the reluctant airlines are making an effort to upgrade. I would agree they should be reluctant. The FAA has encouraged them to move forward in RNP, and other NextGen projects only to loose support after the aircraft were upgraded.

Maybe the FAA will play ball this time.

Load The Bags In The Back

Weight and balance in aircraft is very important. On occasion, you'll see or hear about an aircraft that crashes because of weight and balance issues. Usually the weight is placed too far aft, and suddenly the plane is uncontrollable. Normally it will happen in GA aircraft, but occasionally in transport types there will be too much weight too far aft.

The image above was a 747 where the cargo broke loose after takeoff (or on rotation), and as the aircraft pitched nose up, the cargo slid to the rear. The elevator authority was exceeded, and the pilot was unable to get the nose down before impacting terrain. As the cargo slid aft, the aircraft pitched further up, causing anything loose to go further aft. Eventually the engines couldn't lift the aircraft, so it began falling. 

GA aircraft don't have to have restraints change. So aircraft are tail heavy to start with, and loading things in the rear will make things even worse. Fortunately most aircraft are designed to allow seats to be utilized and some cargo in the rear, and will remain in the loading envelope. 

Most aircraft have a center of gravity (CG) envelope similar to:

Where as the aircraft gets heavier, some weight may not be allowed too far forward or aft (the angles at the top of the graph). The seats are usually in the middle of the graph and the front and rear cargo holds are the front and rear limits of the chart. 

If the aircraft is loaded outside of the envelope, the aircraft will have control issues. Too far aft, and the elevator may not be able to keep the nose down. Too far forward, and the elevator may not be able to lift the nose off the runway. 

There are two points that need to be close to fly stable. The center of lift (where all the lift vectors converge) and the center of gravity (where all the weight vectors converge). If the CG and CoL are exactly at the same point, the aircraft may fly fine. If there is an engine out, with the CoL and CG together, it may be more difficult to glide. Most aircraft fly with the CG ahead of the CoL as a safety precaution. With the CG ahead of the CoL, the nose will drop to help maintain airspeed, and the aircraft will glide nicely with an engine out. 

Moving the CG too far forward will cause extra fuel burn. The elevator will have to push down more to keep the nose up. This pushing down is actually adding to the weight the wings are needing to carry, so the aircraft will have to fly at a higher angle of attack, meaning the aircraft will be extra draggy. 

Aircraft are designed with a specific known airfoil. The airfoil has certain characteristics, and the designer will choose one the meets the criteria of the aircraft. The aircraft will have an optimum load point, which we call the CG. The CG in large transport aircraft the optimum point is measured in a mean aerodynamic chord, or percentage of the wing to operate at. There is a range that is safe, a range that is good and a sweet spot where the is minimum trim drag, or up elevator. 

Most transport aircraft have a sweet spot with a high percentage of the bags in the rear of the aircraft. This will minimize fuel burn, and allow quicker loading of the aircraft. 

FYI  

Electrical System Basics

This post is about electrical system basics, what powers what and why. Not all aircraft have electrical systems, but the ones that do, it is all the same.

In a car, there is a battery, mostly used to start the motor. When the motor runs, there is some excess horsepower used to drive an alternator that is used to recharge the battery and run all the electrical systems in the car. If it didn't have enough power to run everything and recharge the battery, the battery would always be dead.

In an aircraft there are batteries that will start the aircraft. Your normal GA aircraft will probably have a single battery that will be enough to start the motor. Some will be 12V and others 24V. The voltage doesn't matter the concept is the same. The battery starts the motor, and the alternator powers all the other systems while recharging the battery. Light jets are similar, but some will have a starter/generator. The starter once the engine is going will become the generator, to power the electrical systems.

In larger jets, like the 737, there will be a battery, but it will be used to start the APU. The APU is a small engine that turns a generator. The APU will generate enough electricity to start the bigger jet engines. The jet engines have starter/generators on them, that will be used to power the whole aircraft and recharge the batteries.

Many times, jets will use a ground power unit (GPU). This is a cart with an engine and generator or interface to mains power that will be used to power the aircraft while on the ground, and can be used to start the aircraft.

Once the aircraft is powered, there are many systems. These systems can include avionics, entertainment, lighting, etc. Each of these systems will have one or more circuits. Each circuit will be protected with a fuse or circuit breaker (or an electronic equivalent). The circuit protection device is there to prevent fires. The size of the circuit protector is related to the size of the wire going to the circuit. In an aircraft smaller wires mean less weight, so using the right size is critical.

As current flows through a wire, there is resistance. Copper has very low resistance, but not zero. That resistance to the current flowing translates into heat. If the wire cannot dissipate the heat generated, it will transfer the heat to the insulation, potentially melting that. If the insulation melts, and maybe melts the wire next to it, or as the wire passes through a bulkhead and can conduct to the metal airframe, even more current will flow causing even more heat, and maybe something near by will catch fire.

The circuit protector should interrupt the current flowing in the wire before the insulation begins to melt. If the current isn't flowing there is no heat. If there is no heat, there is no fire (or melting insulation causing smoke, etc).

Sometimes things go wrong outside of this. UV rays can cause wire insulation to become brittle (older insulation), and then vibrations would cause the insulation to flake off. This may not be too much of a problem, but it may allow wires to touch, and then the current protection may not work properly. Imagine a 20 amp circuit next to a 5 amp circuit, and the 5 and 20 amp wires touch. The 5 amp fuse will do nothing because the 20amp circuit has taken up the load). Similar problems have happened, and fires break out even though the systems seem to be properly designed.

Should pilots be allowed to switch off circuits in flight? If smoke is in the cockpit, and it seems to be electrical, I believe they should be. Should pilots spend any time troubleshooting trying this circuit or that? I don't think so. If a pilot needs something to complete the flight, they should be allowed to try once, but once a  circuit is turned off, it should be left off. As much as possible, aircraft systems should be designed to enhance the pilots skills, not override it. When things go wrong, pilots need to land as soon as practicable, and have the systems checked out while safely on the ground.

 

AM or FM

For young people, probably born before 2000 (1995?) the idea of AM radio is something they really don't know about. Even in the car, most people don't listen to AM radio any more. The audio quality is much lower on AM than on FM.

AM stands for Amplitude Modulation, FM stands for Frequency Modulation. The modulation has to do with converting a radio frequency carrier into sound. Amplitude Modulation uses the carrier power to cause sound, where Frequency Modulation changes the carrier frequency to cause sound. A radio carrier will typically be a constant frequency and amplitude. A 100MHz radio signal may be broadcast at 100 watts of power. Changes to the carrier at the transmitter can make the receiver change sounds. Changing the amplitude (100 watts +/- 100mw) may make a sound). Changing the frequency (100MHz +/- 20KHz ) will probably also change the sound at the receiver.

Aircraft VHF radios are on AM. Communications with the tower, or aircraft to aircraft is done using AM, like the AM radio in a car. The radios usually work on the 108-140MHz range, which is just above the FM radio in a car (85MHz-108MHz).

The biggest advantage of AM over FM for aircraft communication is when two people try to talk at once. An AM signal of equal strength at the receiver will make a squeal. If one signal over powers the other (IE aircraft is closer to a transmitter) the receiver will hear the closer signal, and may hear the other one as well (possibly making things jumbled). When two FM transmitters of similar strength are received, the radio will only sound like a blank carrier, or white noise. Certainly the receiver will have no idea if someone had a hot mic and sent nothing, or it was two aircraft talking at once.

To change from AM to FM would be a bad idea. People suggest it all the time, the audio in the car shows FM sounds better than FM. The transition would be very expensive, and require all ground stations and aircraft to change on the same day.  AM was chosen because it was easier to work with when aircraft radios were first becoming popular. There would be less clarity on busy frequencies if everything switched to FM.

Newer digital modulation schemes (IE VHDL) will allow more selection. Ground to aircraft radio communications will typically be to a specific aircraft, and prevent the party line that we have today. The party line is helpful, allowing pilots to second guess controller instructions. One pilot may hear a conflicting instruction for two different aircraft, minimizing incidents. The other side of that is sometimes pilots hear instructions meant for another aircraft as instructions for them (IE SWA123 and NWA123 could be on the same frequency).

AM vs FM, what do you prefer?

Talking to Ourselves...

I listen and follow most of the social media, podcasts and some blogs about aviation. Most people are saying the same thing, pilot population is heading down. There are discussions about why this or that, and what laws can be changed to make things better. Some things may help, some may not.

The reality is, we are all talking to ourselves. We talk about flying, and how to get new people involved in aviation. We don't always get to do something about it.

When I was a kid my neighbor was a pilot. He took my dad flying, and his kids flying. I never got to go, but between the stories that my dad told, and his kids told, I really wanted to go flying. I was able to go for rides with my high school buddies, splitting costs and such, but never on my own. I knew I wanted to be a pilot.

My kids tended to be ambivalent towards flying. The first trip we took was in a C-172 from Minneapolis to Billings MT. I don't think I scared them, they were too young to remember how long, hot and awful that trip was (lost an alternator, had a dust storm in Wyoming, and a couple other issues).

Last week I took them to Oshkosh. I think they were just along for the ride to appease me. "Yea, dad we'll go" they kept saying. Getting the car loaded up was difficult, and there was lots of discussions. I wanted to not take them, but that would end my trip, and I wanted to go. I sucked it up, and started driving.

Where we live is close to a largish GA airport. There are plenty of normal looking aircraft, jets, trainers and everything in between. When we got in the gate at Oshkosh, they saw the Cessna and Raytheon display, with all the same aircraft as at our airport. On the right was Aviat and Champs, and others. They were still bored. The next display on the left was the Icon A5, and they just lit up. "Dad, this is the airplane you should buy!". It made me happy that they liked something.

We kept looking, and they stuck their heads in the bomb bay of the B-52, and poked and prodded other aircraft in the main pavilion, and were somewhat interested.  We looked at homebuilts and some of the factory built planes. They weren't there to appease me any more, they were getting something out of this. We went to the Museum and they asked questions about the various airplanes. We got to see the end of Dick Rutan's discussion of the Voyager, and my son went up and talked to Dick. They chatted for a while, and Dick told him the Voyager didn't fly so well, and other things, I didn't hear. Finally he got Dicks autograph, his cherished souvenir.

Then they watched the airshow. My older son saw Mike Gougain perform. He was really impressed, and focused. He asked if I could do that, all I could say is, "I've done some aerobatics, nothing like that".

So then the questions started, what does it take to learn to fly? when can I start? Sunday evening, he turned on the TV and started watching Red Bull Air Races. I will help him get started, but I think he needs to put some effort into making it happen. I'd hate to give him the PPL and have him think that was easy, and only go often enough to be dangerous. He needs to appreciate it.

What can I say, take people to the airport is how to get people interested in flying. Bring 'em into a cockpit. I've taken friends flying,  had boy scouts in my plane, I've flown young eagles, I've done what I could to get as many people thinking positively about flying. If I would have gone to Oshkosh alone, my kids would still be ambivalent, and not thinking about flying. Next time you are going to the airport, even if not to fly, invite someone to come with.

Just Do It!

Twins Really Have 3 Engines; APUs RATs and emergency items

Most twin engine transport aircraft (IE 737, 777, A320, etc) really have 3 engines. The third engine is usually in the tail, and provides almost no thrust. The third engine is generally small (compared to the two engines on the wings) and is called the Auxiliary Power Unit (APU). This APU will provide the aircraft with electricity, air and maybe hydraulic power in the event the wing engines are unable to power the electrical needs of the aircraft.

The APU is a turbine engine, a small jet engine that will provide various resources, like a generator. The APU will use the fuel from the tanks on the aircraft. The engine runs at a constant speed, so there are no throttle controls. There are various gauges to monitor the performance of the engine and the accessories.

The APU can generate enough power to start the larger engines, so it may be running while the aircraft is on the ground. The APU may also provide conditioned air for the aircraft at the gate. The APU can be started using batteries or from the generators on the main engines. Some APUs have separate started batteries from the aircraft other batteries, depending on aircraft needs.

The APU is available for emergency needs. If the aircraft engines are unable to provide air conditioning and pressurization, the APU may be used for that. If the main generators have failed, the APU may provide electricity for the aircraft.

Various scenarios are possible. A maximum performance takeoff will require bleed air from the main engines to be cut off, and the APU can provide pressurization in that case. MELs may allow an aircraft to fly with a single operating generator if the APU is available. Sometimes both generators will fail, and the APU can take the load. For ETOP operations, the APU may need to be running for a portion of the flight (depending on operating limitations).

APUs fail, sometimes. The APU uses fuel, and people are very conscious of fuel consumption, so they are not used all the time. An APU may sit idle for days on certain aircraft, and when they are needed, they don't work, won't start, batteries are dead. The need is still there, so most aircraft have another backup device for emergency electricity generation, called the Ram Air Turbine (RAT).

The RAT is capable of powering enough of the aircraft to get it on the ground. This is a last final device for when things are going bad. The RAT is a propeller attached to a generator that will drop down into the slipstream air close to the fuselage. The electricity will be used to power the pilots PFD and whatever else the pilots need (IE fly by wire system).

The twin aircraft will have two generators, one on each engine. The APU will be there if one or more of those generators fail, and some aircraft have a RAT if all the other generators fail. There is proably no good reason for a pilot not to be able to land a plane if all the electrical systems are out. Will that help you fly more comfortably?