Glass Cockpit Blues

The Square Elephant In The Cockpit

Original Date: June 3, 2009

I was observing on an instrument proficiency check in a Cessna 205 and noticed some things that really did stand out. The pilot undergoing the check was highly competent and ran very thorough checklists for all phases of flight. His VOR and ILS approaches were smooth and safe with limited deflection shown on the CDI that he corrected quickly. However the one instrument in the cockpit that caused the most trouble was the GPS. The instructor asked to see a GPS approach in Orange County. The PIC started pushing buttons to enter approach mode on the receiver. And the GPS promptly decided to ignore his request and do something else, like try to enter an approach for a VOR in the area (which to its credit, it gave a message saying “This is not an airport.”).

So the PIC said lets try a different airport, like Lincoln Park. The instructor said okay, enter the approach and fly the procedure. Again the same flurry of typing and head scratching ensued. By now the instructor is fiddling with the unit and flipping through operation checklists to see if there were any shortcuts to getting it to switch modes. After about 5 minutes he proclaims victory over the beast in the black box and then asks the PIC to enter the approach. The PIC tried several times but each time hit a key that ruined the string of info just entered. That or the wrong option was selected, giving us a flight plan to Aviano. All the while, I’m scanning for traffic and telling the potential student in the back seat next to me that flying is actually fairly easy, but operating the avionics is the thing that makes aces feel like aceholes.

We headed south back to Central Jersey Regional and by this time the PIC had figured out a way to get the GPS to accept the approach mode and left the flight plan mode alone for good. He flew a perfect GPS approach to runway 7, broke off and made a ridiculously soft landing. One of those landings where you have to remind the wheels that they’re supposed to start turning because we are in fact on the ground. After the flight, I talked to the potential student about the joys of general aviation, while the instructor spoke to the PIC about the flight. It was painfully clear that while GPS is a great tool (the map mode would have kept us from guessing where NYC’s class B began in case we couldn’t see ground references, but in that case you should be IFR anyway so it’s a moot point) and it can help you fly more efficiently.

However, if you are not completely comfortable using all modes of the GPS, you’re only getting a fraction of the benefit. Even more importantly, with your head down staring at the various modes on your receiver, you’re distracted from the primary task of flying the airplane. Granted this airplane had an autopilot and it had been used earlier, but the instructor wanted to see the PIC hand fly. The PIC got off heading and altitude far more often when messing with the unit than when he was just scanning the horizon. Granted, a person with an impeccable scan will be able to divide their attention perfectly, but the fact remains that you need to know exactly where the electrons are going before you start the engine.

What's it doing now? Direct ZELEN? I don't even know who ZELEN is!

If your GPS has home training software, use it. Don’t just hit the Direct button and stare at the map. That’s a waste of many thousands of dollars of capability. Practice going to a certain airport and then switching to an alternate. Know how the map orients itself and how to zoom in and out. If your GPS can output commands to an autopilot, do some local practice flights with it engaged in good weather. Basically using the full capability of any avionics needs to be second nature. Just as you can spin the numbers on the transponder without a second thought, so must be the operation of any nav gear.

In closing, a word to any avionics manufacturer who may be reading this (hey you never know). Please make your avionics big enough to use without having to train our fingers how to lock onto the right button while bouncing around in turbulence. Yes, panel space is always an issue but most owners would welcome a large knob that does the same thing in all pages (i.e. scan, change letters, change mode, etc), or large buttons that are spaced so that the bouncing finger doesn’t hit the wrong one. Yes, the “spider crawl” method does work but it freaks out passengers. Other than that one issue, I love the color maps and built in nav/coms. Anything to make the average Piper more like an A320….except for the J-3 Cub.
Let’s leave that one simple.

 

Virtual VFR and Pilot Safety

Don’t Worry, There Won’t Be Any J-3s On An ILS To Minimums

 Original date: August 2, 2011

"I'm sorry sir, these flat panel displays are for airliners only. Have fun with your morse code identifiers."

Warning: This blog is filled with aviation terminology that may be objectionable to land-locked readers.

This all started after reading Mac McClellan’s blog on head-up displays in light aircraft. I posted a response and one of my friends who happens to be an airline captain saw it and responded to my response (don’t you love the internet?). We’ve been going back and forth about the benefits of advanced technology for general aviation aircraft. Specifically, it was about synthetic vision and how it could create Virtual VFR regardless of weather conditions. His stance is that GA pilots don’t need super advanced instruments and information systems because it will make pilots fly into conditions they shouldn’t be in. My stance is that it will make those who take the time to learn how to use it much safer.

The sticking point in any field is that new technology that makes things easier is often seen as a crutch by those who did without for the majority of their lives. When GPS began showing up in aircraft, people said “What will you do if it all fails?” I would then point to their stack of navcoms, adf and loran receivers and ask them the same question. Stuff fails no matter how high tech or low tech it is. Dealing with failures is the burden of the pilot. The mean time between failures with modern electronics far surpasses any analog, transistor or vacuum tube based system that bore the generic label “computerized” in previous decades. Automatically that is a huge benefit not just for safety but for life cycle operating costs.

The other problem brought up during the initial GPS revolution was that people would forget how to navigate or look for other aircraft. That is a problem, not so much of the GPS but of people not knowing how to divide attention, especially in busy airspace. I remember several times with my instructor when we’d spot an airplane (or worse, get bounced from behind) I’d say “Did he even see us?”. To which Marty would always have a witty comeback like “Why don’t you get out and ask him. Think it’ll make a difference?”. I have no idea why the overtakers didn’t see us but a distraction is a distraction. I don’t care if its GPS, an ADF or some poor soul with headphones on listening for “dah-dit dah-dit” on the four course.

Flying in a general aviation aircraft, regardless of what we tell passengers is a more risky activity than driving on average. However the risks can be adjusted based on a pilot’s skill, comfort level and aircraft capability. Maybe a particular pilot doesn’t like flying in clouds, flies only for pleasure and operates an aircraft equipped with VFR only steam gauges. However this pilot wants to upgrade to a 3 tube EFIS system combined with a HUD. Should we deny them advanced navigation and weather information based on the assumption that he is going to suddenly start flying between level 5 thunderstorms? Should information-dense systems be the sole domain of the turbine fleet and business jets? If the light airplane pilot wants to fly a 300nm trip, is it fair to make them use less capable avionics, ostensibly to keep them out of trouble?  

There's a lot of information, but how easy is it to interpret under stress?"

Being able to navigate a couple hundred miles through a high-pressure system without super-duper graphics and satellite weather should not be too difficult for any pilot. A basic GPS or (gasp) a stack of VOR receivers can get you just about anywhere in the United States. But the cushion of safety for those who choose to learn everything that their super-duper system can do for them is undeniable. The objective for VFR pilots is to use extra information to stay away from weather (terrain shouldn’t be a problem since if they’re VFR they should be able to see it). To say they don’t need it because they’ll start flying into frontal systems is like saying that airline pilots shouldn’t have terrain avoidance systems because they’ll see where the ground is and fly into it.

IFR flight on the other hand is a more difficult situation because there are so many variables in the types of aircraft, the types of missions, and the weather conditions at any given place or time. There may be the person in the Cirrus who is cruising at 11,000 feet on top of a cloud layer and wants to know the exact position of the hills hidden beneath those clouds. Sure he can use an IFR chart and know that by staying above the MEA he’ll be fine but let’s use the favorite example of instructors: What if the engine quit? Synthetic vision cannot dead stick an airplane onto an open farmer’s field automatically, but it does give the pilot far more information in an emergency situation with regards to wind direction, terrain location, obstructions, etc.

Take a single-pilot King Air on an ILS on a scuzzy day. The pilot has approach charts that show what the decision height is, how far from the touchdown zone that will be, what the missed approach procedure is, etc. And since the pilot is IFR rated and trains in a simulator at least once a year, it should be no big deal. However, if there is a distraction, or a problem with the aircraft, a small mistake could be made. To the delight of lawyers everywhere, I will be completely honest: pilots do make mistakes (if you don’t believe me, ask the NTSB). The majority of accidents are not one massive brain-fart but a series of smaller errors that compounded until the snowball became an avalanche. By providing easy to interpret data, the pilot’s mind is freed to deal with any other issues that arise during times when the mind is approaching task-saturation. So now while dealing with a generator problem, a sick passenger, or just an unfamiliar approach, the pilot is able to see the image of where the runway should be and cross-reference that with the standard charts and data. This removes all doubt as to the aircraft’s location and where it will be in the next 15 to 30 seconds. Breaking the links in the accident chain should be reason enough for encouraging use of such equipment.

Information-rich technology is not for every style of flying. I admit, it would be odd to fly a Stearman with a HUD. And a Cessna 152 that is only used for $200 dollar hamburger runs (inflation hurts, doesn’t it) would not need an extensive weather suite and electronic IFR charts loaded into the system. Am I in favor of putting EFIS and HUDs in everything from 

Open cockpit EFIS

light sport to piston twins? Honestly it doesn’t matter what I think. If the pilot/operator feels that the technology will be a benefit to their type of flying, then full support should be offered for getting that equipment into their cockpits. I was in Woody Saland’s hangar a while back and was intrigued by the fact that his AirCam had synthetic vision EFIS, EICAS and an autopilot. Why would anyone want so much technology in an open cockpit airplane? Then it hit me: To make the task of converting numbers, radials and performance figures into an instantly interpretable view of what your aircraft is doing. With so much of your mental capacity relieved of that repetitive task, you can actually enjoy the act of flying.

The Case For Long Range Regional Jets

The Case For Long Range Regional Jets

A Practical Application

 

By Christopher Williams

2/6/2010

The regional jet has become a dominant force in air travel in the last decade. While there are economic drawbacks to small jets on short routes, there are decided advantages on longer trips. Because of these advantages, it may be time to move towards long range regional jets. However, before an analysis of a long range regional jet is begun, it is necessary to examine why lower capacity makes sense in not only the traditional hub and spoke system, but in a direct flight system as well.

“Regional jet” is a fairly recent term for a fairly old concept. The Fokker 28 and McDonnell Douglas DC-9-10 would both fit into the category of regional jet (hereafter referred to as RJ) even though neither were consistently used in a manner congruent with current RJs. The modern class of RJs spans the gamut from the 50 seaters such as the Embraer ERJ-145 and Bombardier CRJ200, to the much larger 100 seaters like the Embraer E-190. They are generally powered by high bypass turbofans that have tremendous fuel consumption improvements over turbojets and low bypass turbofans that powered first-generation small jets. Due to their small size, they typically do not have multiple lavatories and baggage storage space is somewhat limited.

Larger airliners (often referred to as “mainline”) have the numbers advantage in terms of seat cost per mile, fuel consumed per seat per unit of distance, or any other statistic involving dividing a quantity by the number of seats. The more passengers that can be carried equates to a lower cost per passenger to operate the flight, which in theory should result in lower fares. RJs are at a disadvantage from the start due to their lower total capacity. Unless total operating costs are reduced by the same factor that the passenger capacity is, there will always be a disadvantage to operating with fewer total seats.

The disadvantage of larger airliners is their higher upfront purchase cost and their higher total operating cost. This is where RJs have the advantage from being smaller aircraft. The aviation consulting group Morten Beyer & Agnew refers to this as the “RJ Operating Cost Paradox”. Even though RJs cost less upfront to buy and less total per hour of operation, they still cost more per person and thus have a much smaller maximum profit margin. [1] This problem has led manufacturers to build larger RJs, like the aforementioned E-190 which boasts up to 114 seats and a breakeven capacity of 61%. [2]

But RJs are not the only aircraft that suffer from reduced capacity. The larger overall cost of operating a full size airliner at reduced capacity is even more of a problem for their operators, which is why so much effort is made to fill every seat on flights. It’s a lot like gambling. With small jets, you take a smaller investment and end up with a smaller return provided you fill your seats. With a big jet, you take a larger investment and end up with a much bigger return, again as long as the seats are filled. If the RJ doesn’t fill its seats, the total loss is still less than a larger jet in the same predicament. Win big, lose big.

                                                                                  
Regional aircraft of varying sizes line up for takeoff at Denver International. Where mainline jets used to do more short to medium range trips, RJs have taken over the lions share of the workload.

How can it be possible combine the advantages of airliners and RJs? The answer may lie with what passengers want rather than what airlines want to give them. For an airline it makes financial sense to use the hub and spoke system to funnel passengers to a few mega-airports and then redistribute them to other smaller airports via RJ or commuter turboprop. For passengers it makes sense to go to an medium sized airport not too far from one’s home and fly direct to wherever they need to go. Unfortunately, unless people live near a large international airport, chances are good that they will have to switch planes at some point, even for a relatively simple trip between terminals of moderate size.

The hub and spoke system was first adapted to passenger aviation by Delta Air Lines in 1955 and later adapted to freight by Federal Express. [3] While this system works wonderfully for packages that have to arrive only by a certain time, say Monday at noon, it does not work well for passengers who want to get to their destination as soon as possible. A package does not care if it sits in a warehouse for 10 hours before being loaded onto a connecting flight to its destination. Passengers tend not to be as enthusiastic about such delays. Another reason for the popularity for hub and spoke had to do with the aerodynamics and performance of first and second-generation commercial jets. With their very high rates of fuel consumption at low altitudes, it was not wise to use them on short trips or at low altitudes. At the time, this had less to do with the price of fuel and more with getting useful range out of the aircraft. To fill the gap, turboprop commuter airliners were pressed into service to connect outlying airports to the major ones. While extremely efficient and fast, passengers still equated them with old lumbering piston airliners. Even though turboprops are technically jet engines with an exposed propeller instead of a shrouded fan, perceptions count for a lot.

RJs are jets through and through which makes passengers happy. But while pilots who may have upgraded from 4 and 6 seat general aviation or charter aircraft might think they’re big, the truth is many passengers still consider them small. And even though newer designs such as the CRJ900 are much larger than the original cadre of RJs, they still cannot compete with the sheer size of an Airbus A340, or Boeing 777. Size aside, the speed and ride quality are on par with small to medium sized airliners. Regional airlines and their supporting major airline counterparts could not buy enough RJs until the fuel crisis of the mid-2000s. At the point when fuel prices skyrocketed, the once beloved aircraft became very unattractive in the eyes of airline accountants. [4] Regional carriers wished for their fleets of turboprops back and many invested in newer designs like the Bombardier Q400 that rival jets with respect to ride quality and sound levels inside the cabin. On short trips, taxi, climb out, descent take up a major portion of a jet’s time and fuel. In some cases the actual time spent at cruise may be around 30 minutes, severely reducing whatever advantages the manufacturer may have designed into the airframe for that portion of flight. In a turboprop the maximum altitude is usually far lower, in addition to the fact that they use a lot less fuel at low altitudes making them more efficient on short trips.

Many people had assumed with the advance of RJs, so too would the ability to travel almost anywhere within reason in the United States without having to go through a hub airport. This never materialized and by 2007, flight delays had risen to a 13 year high as hundreds of RJs tried to occupy the same airspace and parking areas as hundreds of mainline jets. [5] By using RJs to fill in gaps in the hub and spoke system, airlines had unwittingly taken away the major advantage of low capacity jets and undermined their mainline fleets. With the lines blurred between RJ and mainline equipment, some low cost airlines such as JetBlue opted to use A320s for transcontinental and international routes and E-190s for shorter hauls. [6] Oddly enough, successful low cost carriers like JetBlue and Southwest Airlines fly on watered down versions of hub and spoke and use small airliners or RJs on all routes. It remains to be seen if current regional airlines will continue to serve as feeders for the majors or if they will venture off into the volatile world of low cost national operators. Major carriers, currently uninterested in their own RJ fleets, may change their position if that happens.

This next part of this paper will not deal with how to build such an aircraft in detail but will outline the criteria that will be required of the next generation of long range RJs.

Mainline jets, as mentioned, have a huge passenger advantage over RJs. This is also their Achilles heel if the flight is operating at reduced capacity. On red-eyes or on low demand long range routes, using a nearly empty Boeing 757 makes less economic sense than using a partly full Airbus A320. In that respect, using a partly full A320 would make less economic sense than using a nearly full long range RJ. For this to happen, the RJ would have to have transcontinental range with enough reserve to account for average yearly headwinds, missed approaches, a diversion to an alternate field in case of poor weather and any other situation that may stretch the endurance of the craft.

Airliners of all sizes only make money when they fly, and then only if paying passengers are riding inside. Empty seats do not make money which is why full airplanes equal happy shareholders. But passengers do require a bit more room and creature comforts than the average overnight parcel. It is for this reason that future RJs must have as much personal room as the smallest mainline jets. This will require advanced ergonomic design to ensure that the aircraft remains light enough to land at smaller airports, produces minimal drag in cruise but still allows people to stand up in the aisles or go to the bathroom without having to crouch. These same comfort features will be extremely important when considering that the next generation of RJ will have to be able to fly at least 2500nm, if not more.

Takeoff and landing are critical issues for any aircraft and commercial jets have a plethora of criteria to meet before every being certified to carry a single passenger. Airliners do not usually need an entire 10,000 foot runway to takeoff at normal weights, but the extra distance is required in case of an engine failure on takeoff. Below a certain speed (V1) the jet must be able to stop in the remaining runway. Above that speed the airplane must be able to accelerate to takeoff speed in the same remaining runway. For this reason, a Boeing 737 that might be able to become airborne in 3500 feet may require 7000 feet for regulations. This is the balanced field length that is listed as the takeoff distance for all commercial aircraft. Any reduction in takeoff velocity, any increase in acceleration or any combination thereof will go far in reducing balanced field length and thus open up whole new airport markets to airlines. This of course is if there is a demand for the service at those locations.

From takeoff to cruise the aircraft is in a climb. Usually this is not a constant uninterrupted climb as almost all airports have departure procedures (DPs) that require a pause in the ascent at certain points to help with traffic management. Once clear of the immediate area though, most jets are free to climb at their most efficient rate. A major problem for aircraft climbing has not been ability to climb, but excessive noise. Novel concepts for reducing power at strategic segments of the climb such as the Quiet Climb System by Boeing are aimed at making living near an airport quieter. [7] Another approach to the problem is to design aircraft with more excess thrust that allows for steeper climb angles that removes the source of the noise from the ground much faster. An added benefit for pilots and airlines is that the jet will arrive at cruise altitude sooner and begin its most efficient profile earlier in the trip. While excess thrust sounds wasteful, it is only used when required, thus allowing the engines to work at far lower power settings during normal phases of flight, prolonging their lifetimes.

Cruise speeds have been the same for almost all commercial airliners since the 1960s. RJs typically cruised slightly slower than their larger brethren. If the next generation of RJs cruised as fast as or slightly faster than mainline jets, a large advantage would be recognized when combined with the quick climb technique. Current state of the art includes winglets to increase effective aspect ratio of the wing, high bypass turbofans, vortex generators and laminar flow wing sections that all combine to reduce drag.

Descent in commercial aircraft is generally accomplished via flying a standard terminal arrival route (STAR) which is a series of waypoints and altitudes that aircraft follow to remain sequenced and separated on their way to a busy airport in all types of weather. An MIT led study using late night UPS flights into Louisville, KY helped develop the Continuous Descent Approach which saves fuel, time and reduces noise on the terminal approach phase of flight, which often begins over 80nm away. [8] Applying these techniques to an RJ that may even be capable of steeper descents simply means that the high speed cruise portion of flight can last longer and save even more fuel and time overall.

Finally, landing must meet other criteria much like the takeoff had to. Having a low approach speed, strong brakes, thrust reversers (very important on wet, slushy or icy runways as wheel brakes have reduced friction) and aerodynamic devices are all required to make the RJ land in a short distance consistently and safely. The latest in cockpit technology including heads up displays and velocity vector symbology will assist the pilots in using minimal power until the final portion of the approach and touching down exactly where they want to.

Even though flying is the glamorous part that everyone wants to talk about, no aircraft will fly unless it is serviced and repaired on a regular basis. An airplane that is a joy for pilots and passengers will be short lived with airline management if it consumes twice as much in maintenance costs as its contemporaries. Making it easy to work on is another very important aspect. Providing access panels, regardless of material used in construction of the aircraft is very important to maintenance personnel who may have limited time to get an aircraft turned around. Industry standards may apply here as most modern airliners have a 98% or higher dispatch reliability.

When one begins to think of RJs as actual airliners, possibilities for the restructuring of national routes begin to multiply. At this point, the advantages of point-to-point travel begin to make sense and airlines will have the freedom to place their fleet strategically at places that have the demand. If there are seasonal changes, the RJ can much more readily deal with the increase in demand, which may not be enough to warrant using a larger aircraft. It may become possible to fly from a low traffic airport such as Atlantic City, NJ directly to another low traffic airport such as Lansing, MI. How many people actually need to travel to certain city pairs will dictate how often the jets are used. In some cases it may be a once or twice weekly service. In others it may be multiple daily flights. Regardless, the longer segments of the new RJs will make it more economical to use even with a lower total passenger load.

But despite point-to-point being an intriguing idea for travelers, it is not likely that airlines will change their entire route structure overnight. It is therefore important that the new RJs can operate within the current hub and spoke system efficiently as well. This is perfectly feasible provided they are used for the segments that require them. For 65 people who need to travel 1000nm from a hub to an outlying city, they really are not concerned if their airplane is a Boeing 737-800, an Airbus A320 or a CRJ900. What matters to them is that they have practical options for departure and arrival times, that the aircraft is comfortable enough for the stage length and that the aircraft is safe. For pilots what matters is an aircraft that has enough performance to fly the assigned trips without undue effort, that its systems are intuitive and that it is safe. And of course airline managers want it to be inexpensive to purchase, operate and maintain. Having passengers, pilots and maintainers like it only helps to increase the overall value of the purchase.

The maturation of the RJ into an aircraft that is respected and loved by pilots, passengers and airlines is currently taking place. New procedures will have to be developed, new routes will emerge, and new pay scales will have to be developed as airlines integrate long haul RJs into their fleets of mainline jets. Pilots need not look at future RJs as almost-airliners or little airplanes. They will still carry passengers at a significant fraction of the speed of sound several miles above the earth. For those who enjoy flying but wish to spend more time at home with family, flying RJs can offer an attractive option to flying longer international routes. But crew pay must be fair in order to ensure the same caliber of pilot who is flying the Boeing 747-400 over the Pacific is flying the advanced RJ over the Midwest. Murphy’s Law does not care how big an airplane is and sharp individuals are required for all airframes. US Airways flight 1549 (Miracle On The Hudson) is perfect evidence of that.

In today’s economy it has become necessary to rethink everything. That includes how flights are planned, how aircraft are used, how we define what is major, what is national, and what is regional. But it should be remembered that classifications matter not, in the end only the efficient survive. 

 
  
As Bombardier CRJs line up at Philadelphia International, a Boeing 737 taxies past. Perhaps the future of commercial air travel lies in integration for maximum profit and passenger satisfaction.

Works Cited

1. Beyer, Morten & Agnew, Robert. “Morgan Stanley Conference-Regional Jet Update” http://dc228.4shared.com/doc/9I-TFT6r/preview.html

2. “Embraer 190 Specifications”: http://www.embraercommercialjets.com

3. “Delta Through The Decades.” http://www.delta.com/about_delta/corporate_information/delta_stats_facts/delta_through_decades/index.jsp Retrieved: Feb 6, 2010

4. Bachman, Justin. “Airlines Give Propellers Another Spin.” BusinessWeek, April 30 2008 http://www.msnbc.msn.com/id/24390211/page/2

5. Zibel, Alan. “Flight Delays Soar To 13 Year High.” The Washington Post, August 7, 2007 http://www.washingtonpost.com/wp-dyn/content/article/2007/08/07/AR2007080700583.html?tid=informbox

6. “JetBlue and Southwest Airlines Destination Maps”: http://www.jetblue.com/wherewejet/ 

http://www.southwest.com/travel_center/routemap_dyn.html?int=FOOTHOME_WHERE : Retrieved Feb 6, 2010

7. Friedrich, Jerry; McGregor, Daniel; Weigold, Douglas. “Quiet Climb System”. Aero, First quarter 2003: http://www.smartcockpit.com/pdf/flightops/flyingtechnique/25

8. Walton, Jim. “Continuous Descent Arrivals.” 2005 Boeing Performance & Flight Operations Engineering Conference: http://www.smartcockpit.com/pdf/flightops/aerodynamics/5  

 

Light Jet Thrust Formula (#1)

For determining minimum static thrust required for non flat-rated turbofan and turbojet engines.

As temperature and or altitude increases, the amount of thrust produced by an engine will subsequently decrease. On larger turbines, flat rating is available to supply rated thrust above sea level and ISA. For smaller engines that may not have the thermodynamic capacity to flat rate, the designer must take into account the degradation of thrust. Failure to do so will result in unsatisfactory performance, inability to meet expected book values and possible payload restrictions.

The simple fact is as engines get larger, the weight of passengers becomes less of a fraction of the static thrust. Those who wish to build cabin class transportation aircraft are probably going to find their optimum engine (or engines) in the 2000lb thrust class or larger. Use of small jet engines under 500lbs thrust is therefore likely to be associated with very light multi engine craft, experimental/test vehicles and unmanned aircraft. Development of small jet engines however, should not be deterred by market analysis that may not fully understand the different potential uses beyond business and high-end general aviation. Too often advances are cut short by the opposite nature of engineering and profit, which work on completely different time scales.

The urge to use as small an engine as possible should be avoided when building light jet aircraft for the simple fact that passengers cannot be reduced in weight. Smaller engines can operate closer to their optimal TSFC at cruise altitude, but like all of aviation, there has to be a tradeoff for other flight envelopes. Therefore if one wishes to have a 4 place jet with a 200 lb allowance for each passenger, using twin 400 lb thrust engines for a total of 800lbs will provide a 1:1 thrust to payload ratio. This number will gradually change as airframe, fuel, avionics and interior is added. By the time a weight of say 3200lbs is reached (1600lbs empty weight, 800lbs payload and 800lbs fuel…very minimal levels), the total thrust to weight has declined to 4:1. This figure is at sea level on a standard day. Take the same aircraft to Colorado Springs at 65 degrees and the thrust to weight balloons to 5:1. Each pound of thrust has to push over 5lbs of aircraft, thus increasing takeoff distance, time to climb, and fuel consumed over a given stage. Humidity and air pressure may also conspire to rob the engines of performance.

The following formula is very simple as it is meant to be a rough estimate to warn a designer of low thrust levels at a nominal altitude of 6,000 ft MSL and 65 degrees. This altitude also equates to approximately 0.785 with respect to air density at SL ISA. But we’re conservative and/or lazy so the easy to remember 0.75 ratio is the rule of thumb to remember. The thrust lapse rate is based on a low bypass turbofan engine (less than 3:1) and will vary with fan size, nozzle design and turbine temperature. Your engine will vary so consult the appropriate manufacturer tables when advancing beyond the initial planning stage of your design. By using a reduced thrust computation when setting gross weight, unpleasant surprises can be avoided while in the flight test stage.

Tr1/.75=Tr2

Thrust to weight ratio 1 = A/C weight divided by thrust at sea level, ISA.

Thrust to weight ratio 2 = A/C weight divided by thrust at roughly 6000 feet MSL, ISA.

If you really want to scare yourself, multiply 0.75 by the static thrust of your selected engines (assuming they are not flat rated) and see what number you get!