Last Updated: 3 March 2008
MASTER INDEX of articles written, posted online, or recommended by Alex Paterson
NOTE: This is a draft webpage for discussion amongst aviation personnel. As such, it may contain inaccuracies.
The DC9 was an elegantly simple shorthaul jet aircraft designed by the Douglas Corporation in the early 1960s. It was one of the first jet transport aircraft designed specifically to be operated by a two pilot aircrew without the presence of a flight engineer and because of this, considerable effort was put into making the aircraft as simple as possible to operate. From a pilot's point of view the DC9 was a delight to fly, being far simpler and easier to operate than its main rival the B737, which was cobbled together in great haste by Boeing Corporation a couple of years after the production of the first DC9 in response to the phenomenal success of the DC9.
In the late 1990s Boeing Aircraft corporation bought out the trouble McDonnell-Douglas company (the manufacturer of DC9s and its derivatives). Boeing subsequently made a decision to continue manufacturing a DC9 derivative called the B717. Below is a cursory comparison between the modern 21st century B717 and the 40 year old DC9 it is based on.
Interestingly enough, the B717 contains a number of fundamental design faults, which in the opinion of this reviewer, is a disgrace considering the aircraft is derived from such a well designed aircraft in the first place. The following comparison provides a stark insight into how the current transport jet aircraft manufacturers, Boeing and Airbus, have forgotten the most important (and fundamental) design imperatives pertaining to civilian jet transport aircraft, those being simplicity of design and sound ergonomics. 1
It is the view of many pilots that both Boeing and Airbus have lost sight of the fact that a simple aircraft in the hands of skilled, well trained pilots is generally speaking a far safer aircraft to operate than a highly automated, complex aircraft allegedly designed to make up for the deficiencies of poorly skilled pilots. Many aviation professionals (including the author) predict a spate of avoidable aircraft disasters over the next 20 years associated with the ever increasing complexity of transport jet aircraft.
REMOVED in the B717. Replaced by a computer menu on one of centre flat panel screens.
The DC9 had a 'pull out and up' mechanical checklist located on top of the glareshield in front of the centre windscreen. (i.e. above the autoflight mode control panel) The advantages of such an arrangement are numerous:
By contrast, electronic checklists are generally located at the bottom of the center instrument panel, requiring pilots to look down into the cockpit at crucial times, such as during the approach to land. Indeed, the lower center-panel location can be dangerous even when the airplane is taxiing - as evidenced by the case of a jet that collided with a truck in the terminal area partly due to the fact that the pilots were performing the checklist at night whilst taxiing to the gate.
Assessment: A potentially dangerous negative compared to the DC9. Checklists on the B717 are now performed on an electronic checklist where the pilots are required to look down into the cockpit instead of up and outside where their focus of attention should be.
Removed on the B717 and replaced by an electronic Engine Indication and Crew Alert System (EICAS) located on the Centre Forward Instrument Panel.
An 'Annunciator Panel' consists of an array of warning lights used to advise pilots as to the status of various aircraft systems. On the DC9 the Annunciator Panel was located on the forward overhead panel just above centre windscreen.
As mentioned above the Annunciator Panel was removed on the B717 and replaced by an electronic Engine Indication and Crew Alert System (EICAS) located on the Centre Forward Instrument Panel. Presumably Boeing would argue there some advantages in providing an EICAS as opposed to a simple Annunciator Panel of color coded lights with words in them, as more information can be provided to crew members during abnormals and emergencies. However, in the opinion of many pilots (including the author) the DC9 and the B717 are such simple aircraft they do not really warrant an EICAS.
As with electronic checklists (see above), the location of the EICAS screens on the B717 requires pilots to look down into the cockpit to get information about systems whose switches are located up in the overhead panel. This arrangement forces pilots to look down into the cockpit to read about a system emergency or abnormal and then up at the overhead panel to action the said instructions, a procedure that has the potential to induce pilot disorientation (vertigo), especially at night and/or in bad weather. By contrast, the DC9 Annunciator Panel was located such that both pilots' peripheral vision remained out the windscreen (where their eyes should be most of the time anyway) when looking at the Annunciator Panel and on the way up to the relevant system switches located on the overhead panel.
NOTE: Boeing is following the current trend of using computer screens to replace a simple array of warning lights in an effort to 'modernise' aircraft. Having made that decision, Boeing had the opportunity to embrace the best of both worlds by splitting the functions of the Engine Instruments and Crew Alert System (EICAS) into their separate components, with Engine Instruments remaining on the centre forward panel where they have always been traditionally located and re-locating the 'Air Crew Alert System' (ACAS) to where the old DC9 Annunciator panel is presently located. (i.e. on the forward overhead panel just above centre windscreen)
Assessment: In many respects a computer generated Air Crew Alert System (ACAS) is a potentially dangerous irrelevancy on a simple shorthaul aircraft such as the B717. The DC9 annunciator panel (consisting of system warning lights) provided all the information a competent aircrew needed to know about system status. Detailed instructions on a computer screen describing an emergency is not the time, nor place for pilots to learn about their aircraft systems as they should already have a comprehensive understanding of how the aircraft system works prior to flight. ACAS on an aircraft as simple as the B717 has the potential of encouraging pilots to just "blindly follow the computer" instead of thinking about the action they are about to perform. 2
For more on the dangers of blindly following the computer see:
RETURN TO B-717 PHOTO
Removed on the B717 and replaced by a poorly located electronic computer generated screen requiring electrical power at all times.
On the DC9, the Standby Attitude Indicator (AI) was located one instrument removed to the right of the Captain's Attitude Director Indicator (ADI). The standby AI was a Sfena electro-mechanical AI, which would continue to provide reliable attitude information for up to 5 minutes after the removal/loss of electrical power to it.
By contrast the B717 has a Standby AI located at the bottom middle of the centre instrument panel underneath the Engine Instruments and Crew Alert System (EICAS) screens, a position far removed from the notice of either pilot. The B717 Standby AI is a computer generated display, instead of the electro-mechanical unit fitted on DC9s, meaning that it must be powered by electricity at all times to operate.
NOTE 1: The DC9 can be flown quite safely without any electricity, with the Sfena electro-mechanical Standby AI providing accurate attitude information for up to 5 minutes following the removal or loss of electricity. The other main flight instruments were mechanical, meaning Airspeed and Altimeter information did not require any electricity to operate. This is of particular relevance in the case of an inflight electrical fire as both the B717 and DC9 do not have an effective strategy in place to handle in-flight electrical fire. The option of turning off all electrical power was (and remains) a realistic option for dealing with an in-flight electrical fire on the DC9. (see also Mechanical Outflow Valve item below for more on the issue of in-flight electrical fire)
NOTE 2: Interestingly enough, there appears to be plenty of room to the left of the Captain's main ADI (and to the right of the FO's main ADI) to accommodate a mechanical Sfena Standby ADI, mechanical Standby Altimeter and mechanical Standby Airspeed indicator.
NOTE 3: ALL jet transport aircraft should have at least one (and preferably two) electro-mechanical Standby Attitude Indicators, mechanical Standby Altimeters and mechanical Standby Airspeed Indicators, located next to their respective Primary instruments, if only to inspire pilot confidence. The electro-mechanical Sfena Attitude Indicator (AI) rarely gave trouble.
Assessment. A seriously dangerous negative. The design and placement of the B717 Standby AI completely fails to meet the most important purpose of a Standby AI, that being an instantly noticeable crosscheck for the Captain's ADI. The twinning of the primary ADI and its backup should be a "sine qua non" of good cockpit design.
RETURN TO THE B-717 PHOTO
The B717 has the airspeed and altimeter incorporated in each pilot's Attitude Director Indicator screen as electronic vertical 'tapes' either side of the attitude indicator. Whilst such an arrangement may be appropriate for an Altimeter, it is not appropriate for an Airspeed Indicator. Round analogue gauges arcing through at least 270 degrees provide an instantly noticeable 'picture' with regard to airspeed. This is especially important with regard a sudden decay in airspeed associated with windshear. The actual airspeed 'numbers' themselves are fairly irrelevant; it is the position of the airspeed indicator needle in relation to the orange datum 'bug' that is important.
Assessment: The vertical airspeed tape used in the B717 provides pilots with an inferior and less intuitive presentation of airspeed than the round Analogue Airspeed Indicator it replaced. Pilot misinterpretation of the airspeed tape at critical stages of flight such as takeoff and landing has the potential to be a serious pilot trap.
RETURN TO THE B-717 PHOTO
Removed on the B717 and replaced by an electrical switch on the overhead panel which is neither intuitive to find nor use and requires electricity to depressurize the aircraft.
The DC9 had a mechanical lever located on the centre consol next to the Captain's seat which mechanically operated a manual outflow valve located just aft of the cockpit. This lever was both intuitive for pilots to grab and to operate in order to depressurize the aircraft especially if they had been dazed in a crash. More importantly, the mechanical lever required no electricity to operate the standby outflow valve.
Assessment: Yet another example of Boeing removing an intuitive and tactile mechanical control and replacing it with a less intuitive, harder to find, electrical switch to operate a more complicated system requiring electricity to operate it. The fact that the B717 system requires electricity to operate it is a potential death trap in the event of a serious electrical failure as none of the aircraft doors can be opened unless the aircraft is first depressurized.
NOTE: The world's sixth worst air disaster of all, which happened at Riyadh Airport Saudi Arabia in August 1980, is thought to have been caused by an electrical fire which rendered the aircraft unable to be depressurized. The Saudia Arabian L-1011 had been successfully landed with an electrical fire, but because it couldn't be depressurized, none of the doors could be opened and all 301 passengers and crew perished. The L-1011 has no mechanical outflow valve in which to depressurize the aircraft without electricity.
Same as the DC9. Engine Fire Handles are located in the best place possible, that being on the Centre Forward Instrument Panel just above the Engine Indicator and Crew Alert System (EICAS) screen where they remain within the peripheral vision of both pilots when flying on instruments.
It would be interesting to see if Boeing have complicated the drill by incorporating the standard Boeing Engine Fire drill of instructing pilots to:
instead of the old DC9 drill of the pilot flying simply reaching forward and, on confirmation from the support pilot, pulling the illuminated fire handle.
RETURN TO THE B-717 PHOTO
The DC9 was a simple aircraft to operate and fly, with very simple straight forward systems. It was considered by most pilots who flew it as being "a pilot's aircraft" and as such a joy to fly. It could be easily and safely flown without any hydraulics or electrical power. It would appear that Boeing have needlessly made an elegantly simple aircraft more complicated (and less safe) in an endeavour to 'modernise' it. 3
Definition: the study of the efficiency of persons in their working environment. (Source: Oxford Dictionary) With regard to aircraft, good ergonomics implies that aircraft should be designed in such a way that pilots have a clear view of aircraft performance and system status, and it should not contain what might be called 'pilot traps.' Traps are hereby defined as design features or aspects that tend to confuse pilots about an unfolding event, and which can distract them or encourage them into making inappropriate (and by implication, dangerous) decisions.
2. EMERGENCY and ABNORMAL CHECKLISTS
Emergency checklists are best
served by a dedicated book (red) using large print designed to be
read at night in poor light. All emergency drills should be as simple
Abnormal Items checklist should be listed by system in a separate book. (Yellow)
Performance data should be located in a separate Quick Reference Performance handbook or plasticised card.
Boeing's Quick Reference Handbook (QRH) is a confusing mess as it contains Emergency items, some Abnormal items, some Performance Data and some Normal Check lists. (i.e. it is a jack of all trades and a master of none)
3. ELEGANT is defined as "ingeniously simple and pleasing". (Source: Oxford Dictionary 1994)
The DC9 is certainly that.
Copyright © Alex Paterson 1999
MASTER INDEX of articles written, posted online, or recommended by Alex Paterson
ABOUT Alex PATERSON
Alex PATERSON is an Australian airline pilot. He writes articles and advises on issues pertaining to aviation, politics, sociology, the environment, sustainable farming, history, computers, natural health therapies and spirituality.
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The document, 'B717 - DC9 Comparison' is the copyright © of the author, Alex Paterson. All rights reserved by the author. Not withstanding this, the document may be reproduced and disseminated without the express permission of the author so long as reference to the author is made, no alterations are made to the document and no money is charged for it.
Additional Keywords: Attitude Director Indicator, ADI, RMI, B717, DC9, ergonomics, aviation safety, airline safety, mechanical checklists, annunciator panel, engine fire handles
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