* While Canada is not generally perceived as a center of the aircraft industry, in reality the country is a prominent player in commercial aircraft through the Bombardier Group. One of the major components of Bombardier's product line is the family of business jets and light airliners established with the Canadair Challenger and its derivatives, the Canadair Regional Jet (CRJ) and Global Express. This document provides a history and description of the Challenger, CRJ, and Global Express series of aircraft.
* What would become the Canadair Challenger was designed by Bill Lear of Learjet fame in the mid-1970s. Lear had sold off his aircraft production facility to the Gates Rubber Company to focus on design through his Lear Avia company, and began to peddle his "LearStar 600" concept to various aviation firms. Canadair bought rights to the proposal in 1976, but insisted on a wider fuselage -- 2.74 meters (108 inches): marketing studies had shown customers wanted to have "stand up" room in a business aircraft.
Lear objected, feeling with legitimate reason that it would degrade the lines of the aircraft, calling the Canadair modification "Fat Albert". Canadair management compromised, inviting Lear to submit his own derivative design with a wider fuselage -- but to no great surprise, in the end Canadair preferred their own derivative over Lear's "Allegro". Canadair's push for a widebody business jet would prove justified over the long run. Business jets to that time had been cramped, and the roominess provided by the expanded fuselage more than compensated for its fat lines. It would also provide a strong platform for expansion.
The first of three prototypes of the initial production version, the "Challenger 600" or "CL-600", performed its initial flight on 8 November 1978, with test pilot Doug Atkins at the controls. The first prototype was later lost in a crash on 3 April 1980, three crew successfully bailing out but the captain being killed. The type obtained certification in late 1980.
The Challenger 600 provides a baseline for description of the Challenger family. It was a business jet of generally conventional configuration, with all swept flight surfaces, including a low-mounted wing and a tee tail. It had tricycle landing gear, with all gear assemblies featuring twin wheels. The nose gear retracted forward and the main gear hinged from the wing near the wing root in towards the fuselage; incidentally, there were no covers over the main gear wheels. It was powered by twin Avco Lycoming ALF-502L turbofans, mounted on the rear of the fuselage, each engine providing 33.6 kN (3,400 kgp / 7,500 lbf) thrust. The Challenger 600 had a crew of two and could carry from 14 to 18 passengers, depending on seating arrangement.
CANADAIR CHALLENGER 600: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 18.85 meters 61 feet 10 inches wing area 41.8 sq_meters 450 sq_feet length 20.85 meters 68 feet 5 inches height 6.3 meters 20 feet 8 inches empty weight 10,285 kilograms 22,675 pounds MTO weight 18,200 kilograms 40,125 pounds max cruise speed 890 KPH 550 MPH / 480 KT service ceiling 13,720 meters 45,000 feet range 6,300 kilometers 3,915 MI / 3,400 NMI _____________________ _________________ _______________________
84 Challenger 600s were delivered into 1983. Most were later upgraded with winglets to the "Challenger 600S" configuration. The Canadian Armed Forces obtained a dozen Model 600s, with seven configured as "CE-144" electronic support and training aircraft; one used as a "CX-144" electronic test and trials platform; and four used as "CC-144" VIP transports. All twelve were later upgraded with winglets.
* The ALF-502 turbofans proved a serious weakness of the Challenger 600; they weren't powerful enough, and apparently were not all that reliable. That mean fitting new engines, in the form of the General Electric (GE) CF34 turbofan, leading to the "Challenger 601" series, which included:
Four Model 601s were acquired by the Canadian government as VIP transports, once again being designated "CC-144". The type was also obtained in small numbers by the German Luftwaffe, the People's Republic of China, and Malaysia.
* Canadair was acquired by the Bombardier group in 1986; the Challenger program continued energetically under new management. The 601 series was followed by the "Challenger 604" series. The program was initiated in early 1993, with the initial flight taking place on 18 September 1994. The 604 featured additional fuel tankage in the rear fuselage; a stronger undercarriage to handle higher take-off weights; a stronger tail unit; new wing root fairings; improved GE CF34-3B engines with 28.84 kN (3,960 kgp / 8,730 lbf) thrust each; and a Collins Pro Line 4 avionics suite with large electronic flight instrumentation system (EFIS) displays. FAA certification was awarded in late 1995, with first deliveries taking place the next year, in 1996.
BOMBARDIER CHALLENGER 604: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 19.61 meters 64 feet 4 inches wing area 45.71 sq_meters 492 sq_feet length 20.85 meters 68 feet 5 inches height 6.3 meters 20 feet 8 inches empty weight 9,805 kilograms 21,620 pounds MTO weight 21,865 kilograms 48,200 pounds max cruise speed 870 KPH 540 MPH / 470 KT service ceiling 12,500 meters 41,000 feet max takeoff length 1,740 meters 5,700 feet range 7,550 kilometers 4,690 MI / 4,080 NMI _____________________ _________________ _______________________
In 2005, Bombardier began development of the "Challenger 605", much like the C604, but with a Rockwell Collins ProLine 21 avionics suite; a redesigned interior with bigger windows, shifted up the fuselage; and a tail cone instead of a boat tail. Initial flight was on 22 January 2006. It obtained certification in late 2006 and went into service in 2007.
The Challenger 605 was followed by the "Challenger 650", featuring a redesigned and more spacious cabin; GE CF34-3B MTO engines with 41 kN (4,180 kgp / 9,220 lbf) thrust each; and a state-of-the-art "glass cockpit" scheme named "Bombardier Vision". The Challenger 650 was announced in 2014, unveiled to the public in the spring of 2015, with shipments beginning late in that year. The Challenger series has been a money-maker for Canadair / Bombardier, with well over a thousand sold to date.BACK_TO_TOP
* In 1995, Bombardier began studies for a business jet positioned between the Learjet and the Challenger 600 series, with a cabin mockup of what would become the "Challenger 300" or "Challenger Continental" displayed to the public at the National Business Aircraft Association (NBAA) Convention in Las Vegas in 1998. Initial flight of the prototype was on 14 August 2001 from the Bombardier Learjet plant in Wichita, Kansas, with test pilot Jim Grabman at the controls.
The second machine performed its initial flight on 9 October 2001, with Grabman and Doug May at the controls. The third aircraft performed its first flight on 7 December 2001, with the fourth -- the first with a complete interior -- following on 5 April 2002, and the fifth on 8 March 2003. Along with the five flight test machines, two static test airframes were built as well. Type certification was granted in 2003, with initial production rolled out at the same time. The Bombardier Flexjet fractional ownership program was the first user, with initial customer flight on 8 January 2004.
The Challenger 300 was designed to provide a transcontinental transport capability for a typical load of eight passengers, though high-density arrangements for 15 are possible as well. It is not simply a modified Challenger 604/605, instead being essentially a new aircraft, only featuring a general layout similar to that of the other Challengers. The Challenger 300 features a Rockwell Collins ProLine 21 avionics suite and glass cockpit. It is powered by twin AlliedSignal (Honeywell) AS907 turbofans with a thrust of 30.4 kN (3,095 kgp / 6,825 lbf) each and full authority digital engine controls (FADEC).
BOMBARDIER CHALLENGER 300: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 19.23 meters 63 feet 1 inch wing area 48.5 sq_meters 522 sq_feet length 20.97 meters 68 feet 10 inches height 6.22 meters 20 feet 5 inches empty weight 10,140 kilograms 22,350 pounds MTO weight 17,010 kilograms 37,500 pounds max cruise speed 870 KPH 540 MPH / 470 KT service ceiling 13,715 meters 45,000 feet max takeoff length 1,450 meters 4,755 feet range 5,740 kilometers 3,565 MI / 3,100 NMI _____________________ _________________ _______________________
Bombardier announced an improved variant, the "Challenger 350", in 2013, with the first performing its initial flight on 2 March of that year, and initial deliveries in 2014. It is difficult to distinguish from the Challenger 300, the primary change being Honeywell HTF7350 turbofans, providing better fuel economy, power, and range -- extended to 5,925 kilometers (3,680 miles / 3,200 NMI). It also featured reprofiled winglets, and a redesigned interior.BACK_TO_TOP
* The wide fuselage of the Challenger suggested early on to Canadair officials that it would be straightforward to stretch the aircraft to provide more seats, and there was a plan for a "Challenger 610E", which would have had seating for 24 passengers. It didn't happen, the effort being canceled in 1981 -- but the idea didn't go away, either.
In 1987, studies began for a much more ambitious stretched configuration, leading to the formal launch of the "Canadair Regional Jet (CRJ)" program in the spring of 1989. The "Canadair" name was retained despite the fact that Bombardier had bought out the company. The first of three development machines for the initial "CRJ-100" performed its first flight on 10 May 1991, though one of the prototypes was lost in a spin mishap in July 1993. The type obtained certification in late 1992, with initial delivery to customers late in that year.
The CRJ-100 was stretched 5.92 meters (19 feet 5 inches), with fuselage plugs fore and aft of the wings, and featured two more emergency exit doors, plus a reinforced and modified wing with extended span. Typical seating was 50 passengers, the maximum load being 52 passengers. The CRJ-100 also featured a Collins ProLine 4 avionics suite, Collins weather radar, GE CF34-3A1 turbofans with 41.0 kN (4,180 kgp / 9,220 lbf), more fuel capacity, and of course improved landing gear to handle the higher weights. It was followed by the "CRJ-100ER" subvariant with 20% more range, and the "CRJ-100LR" subvariant with 40% more range than the standard CRJ-100.
The CRJ-100 series was then replaced in production by the "CRJ-200", which was much the same, except for fit of GE CF34-3B1 turbofans, with the same thrust levels as the CF34-3A1 but improved fuel consumption. Initial deliveries were in 1996. Of course, "CRJ-200ER" and "CRJ-200LR" longer-ranged variants were introduced, as was a business jet variant, somewhat confusingly known as the "Challenger 800", later refined into the "Challenger 850".
BOMBARDIER CRJ-200ER: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 21.21 meters 69 feet 7 inches wing area 48.35 sq_meters 520.4 sq_feet length 26.77 meters 87 feet 10 inches height 6.22 meters 20 feet 5 inches empty weight 13,740 kilograms 30,290 pounds MTO weight 23,135 kilograms 51,000 pounds max cruise speed 860 KPH 535 MPH / 465 KT service ceiling 12,500 meters 41,000 feet takeoff field length 1,525 meters 5,010 feet range 3,045 kilometers 2,645 MI / 1,645 NMI _____________________ _________________ _______________________
* Over 400 CRJ-100/200 machines had been delivered by the end of the century. By that time, a further stretched variant with a capacity of 70 passengers, the "CRJ-700", was in advanced development.
Preliminary investigation for the 70-passenger CRJ-700 began in 1995, with formal program launch in 1997. Flight of the initial development machine was on 27 May 1999, with test pilot Craig Tylski at the controls and assisted by copilot Chuck Ellis. Certification was in late 2000, with initial customer deliveries in 2001.
The CRJ-700 is powered by twin CF34-8C1 turbofans with 56.4 kN (5,745 kgp / 12,670 lbf) thrust each and FADEC. It features a new wing with an increased wingspan and wider tailplane as well as a roomier interior with a lower floor. While the usual seating arrangement is for 70 passengers, arrangements for 72 or 78 passengers are possible as well. The cockpit is fitted with a Rockwell Collins ProLine 4 avionics system with six displays. The product line also includes the "CRJ-700ER" and the "CRJ-700LR".
BOMBARDIER CRJ-700: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 23.24 meters 76 feet 3 inches wing area 70.61 sq_meters 760 sq_feet length 32.51 meters 106 feet 8 inches height 7.57 meters 24 feet 10 inches empty weight 19,595 kilograms 43,200 pounds MTO weight 32,885 kilograms 72,510 pounds max cruise speed 860 KPH 535 MPH / 465 KT service ceiling 12,495 meters 41,000 feet max takeoff length 1,565 meters 5,135 feet range 3,150 kilometers 1,135 MI / 1,700 NMI _____________________ _________________ _______________________
A 75-seat "CRJ-705" with an improved interior was introduced in 2005.
* In 1999, confronted with the introduction of higher-capacity regional jets from competitors, Bombardier began to investigate an all-new aircraft, the "BRJ-X", as well as a further stretch of the CRJ-700 designated the "CRJ-900", with a maximum capacity of 90 passengers. The BRJ-X concept remained in limbo, but Bombardier formally committed to the CRJ-900 in July 2000.
The first prototype, which was a modification of the prototype CRJ-700, performed its first flight on 21 February 2001. It retained the wings, engines, wheels, and brakes of the CRJ-700. The first new-build CRJ-900 performed its initial flight on 20 October 2001. The type obtained certification in 2002 and initial deliveries to customers were in 2003.
The CRJ-900 actually can't quite carry 90 passengers, there being 86 seats in a high-density configuration. Other configurations include an 80-seat single-class arrangement and a 75-seat two-class arrangement, with 15 seats in business class. The CRJ-900 is powered by twin GE CF34-8C5 turbofans with a normal take-off thrust of 58.4 kN (5,950 kgp / 13,125 lbf), and features a stronger wing and more robust landing gear. New overwing exits were added on each side. Both ER and LR variants are available.
BOMBARDIER CRJ-900: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 24.85 meters 81 feet 6 inches wing area 70.61 sq_meters 760 sq_feet length 36.40 meters 119 feet 4 inches height 7.51 meters 24 feet 7 inches empty weight 21,545 kilograms 47,505 pounds MTO weight 36,514 kilograms 80,500 pounds max cruise speed 880 KPH 550 MPH / 475 KT service ceiling 12,495 meters 41,000 feet max takeoff length 1,875 meters 6,145 feet range 2,955 kilometers 1,555 MI / 1,350 NMI _____________________ _________________ _______________________
The CRJ-190 was followed by the 100-seat "CRJ-1000", which has a has a fuselage stretch of 3 meters (10 feet). It retains the same engines as the CRJ-900 and has about 90% parts commonality, with a maximum takeoff weight of 41,630 kilograms (91,800 pounds) and a range of kilometers (1,691 NMI). It has larger overhead storage bins and windows, as requested by customers. Initial flight was in the summer of 2008, with certification and initial customer deliveries in late 2010.
* Beyond the CRJ-1000, Bombardier is working on a "CSeries" with a different configuration, featuring twin underwing turbofans. The effort was "touch & go" for a time, with the program suspended in early 2006, to be revived in a year later. The company is currently developing a 110-seat "CS100" and a 130-seat "CS300". They will be powered by the new, highly efficient Pratt & Whitney PW1000G geared turbofan and will make extensive use of composite materials in their construction. Initial flight of the CS100 was on 16 September 2013,; initial flight of the CS300 was on 28 February 2015, with both aircraft to enter service in 2016.BACK_TO_TOP
* While building up the CRJ line, Bombardier also pursued development of a high-end, transcontinental-range business jet, the "Global Express". The program was publicly unveiled in 1991, with the first of four flight-test prototypes performing its initial flight on 13 October 1996. Two static test prototypes were also built. The type was certified in 1998, with initial deliveries to customers by the end of that year.
The Global Express has the same general configuration as the CRJ series, but it is a new-design aircraft, in particular featuring a new wing. It is powered by twin, tail-mounted BMW / Rolls-Royce BR710A2-20 turbofans with FADEC and providing 66.1 kN (6,690 kgp / 14,750 lbf) thrust. The cockpit is fitted with a Honeywell Primus 2000 flight system with six displays. Flightcrew includes pilot, copilot, and one or two flight attendants. The number of seats normally ranges from 8 to 18.
BOMBARDIER GLOBAL EXPRESS: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 28.60 meters 94 feet wing area 94.9 sq_meters 1,022 sq_feet length 30.30 meters 99 feet 5 inches height 7.57 meters 24 feet 10 inches empty weight 22,135 kilograms 48,800 pounds MTO weight 42,410 kilograms 93,500 pounds max cruise speed 935 KPH 580 MPH / 505 KT service ceiling 12,495 meters 41,000 feet max takeoff length 1,690 meters 5,550 feet range 12,405 kilometers 7,705 MI / 6,700 NMI _____________________ _________________ _______________________
* The Global Express has been followed by derivatives. The "Global 5000" is effectively a slightly "shrunk" version of the original Global Express, with the fuselage shortened by 80 centimeters (31.5 inches) and less fuel capacity, giving a maximum range of 8,890 kilometers (4,175 miles / 4,800 NMI) with eight passengers at economical cruise speed. The Global 5000 program was announced in 2001. The first of two prototypes performed its initial flight on 7 March 2003, with Craig Tylski and Gary Bruce at the controls, accompanied by flight test engineer Scott Runyan. The type entered service in 2005.
The "Global Express 6000" -- originally the "Global Express XRS" -- in contrast, has slightly greater range than the original Global Express. It features a new fuel tank in the wing-body fairing, as well as a rapid refueling capability; a modification to the flap system to permit shorter takeoffs; two more windows and a bigger storage space; and standard fit of the "Bombardier Enhanced Vision System (BEVS)" head-up display system. It is otherwise similar to the original Global Express. The Global Express 6000 was announced in 2003, with initial flight of the first prototype on 16 January 2005. The Global Express 6000 entered service in 2005, and has now replaced the original Global Express in production.
A "Global 7000" will be introduced in 2016, to be followed by a "Global 8000" in 2018, these aircraft having more cabin space, longer range, better fuel economy, and other improvements.BACK_TO_TOP
* Bombardier jets are in extensive military service, most significantly in the form of VIP transports -- but also in more aggressive roles.
Three Challenger 604s were obtained by Denmark as coastal patrol and utility aircraft, and South Korea has also obtained the type for coastal patrol. The Danish machines, which were converted to the "Multi-Mission Aircraft (MMA)" configuration by Field Aircraft, have a AN/APS-143 search radar in a fairing in the belly near the leading edge of the wing, and an electro-optic ball turret that retracts into the belly, positioned between the engines. One had an external attachment point for a side-looking radar or a weapon; all three had "quick change" interiors to allow them to be used as VIP aircraft.
A fourth was configured solely as a VIP aircraft. The Danish patrol machines are now to be upgraded, with a new radar, datalink, and modernized cockpit. The upgrade is to be completed by 2018. The configuration of the South Korean machines is unclear, though they do have a radar fairing and other protuberances suggesting sensor fits.
In 2013, Boeing announced the development of a "Maritime Surveillance Aircraft (MSA) based on the Challenger 605 as a relatively low-cost solution for the maritime patrol mission. Mission systems will be in part derived from the Boeing 737-based P-8 Poseidon maritime patrol aircraft, most notably the Poseidon's AN/APY-10 multimode radar and its integrated software system. The antenna fairing, however, will be larger than it is on the Danish MMAs, allowing fit of other radars as per customer requirement. Flight crew will include two pilots and from three to five systems operators, each with a workstation. Mission endurance will be up to eight hours, with six hours on station.
Boeing is working with Field Aviation for the MSA conversions, with Field obviously leveraging off the Danish MMA conversions to kit up a Challenger 604 as an MSA demonstrator, with Field flying a demonstrator from early 2014. No sales have been announced yet.
In the spring of 1999, Raytheon was awarded the contract in the $1.3 billion USD "Airborne Stand Off Radar (ASTOR)" competition for the UK. The ASTOR program was intended to develop a surveillance aircraft carrying air-to-ground radar for battlefield surveillance. ASTOR was a joint program of the British Army and Royal Air Force (RAF). The program had its origins in the 1980s in the form of the "Corps Airborne Standoff Radar (CASTOR)" requirement for the British Army, which wanted to obtain the capability to observe Soviet military movements in Central Europe. The fall of the Soviet Union threw CASTOR into confusion, but the US Joint-STARS radar platform, based on the Boeing 707 jetliner, proved very useful during the Gulf War in 1991; the British were impressed, leading to the issue of the ASTOR requirement in the mid-1990s.
The UK obtained five ASTOR systems based on modified Global Express airframes, along with two portable ground sites and six tactical ground stations mounted on trucks. The primary payload was a "synthetic aperture radar (SAR)" system based on the Hughes "Advanced SAR Radar Type 2 (ASARS-2)" radar system, used on the US Air Force U-2 reconnaissance aircraft. The ASTOR had a set of radio datalinks to allow it to interact with ground stations. The aircraft could operate at altitudes of 15,250 meters (50,000 feet) or higher for 11 hours. Initial plans to add inflight refueling capability were canceled. The aircraft had two pilots and three electronics system officers, and additional seats and bunks for relief crews.
A Global Express was fitted with equipment fairings for aerodynamic tests, performing its initial flight in this configuration in 2001. An operational prototype with a full gear fit was flown in 2004 and delivered to the UK for service trials, with the type being accepted into service in late 2008 and promptly sent off to Afghanistan. In RAF service, the ASTOR was given the designation of "Sentinel Reconnaissance Mark 1" or just "Sentinel R1".
Due to UK budget shortfalls, the decision was made to axe the Sentinel fleet after its return from Afghanistan, with the aircraft to remain in service until they could be replaced by a cheaper alternative. However, the Sentinels also proved useful in the Anglo-French intervention in Mali in 2013, with the RAF requesting that the cancellation of the program be reconsidered. With the Sentinel proving its value in other foreign interventions, as well as in monitoring flooding in the UK, the government agreed to keep the Sentinel in service until at least 2018. There's even been consideration of upgrades, most prominently a software change that would give the Sentinel a secondary maritime search capability.
* The US Air Force has also obtained the Global Express for battlefield operations, as a flying communications platform designated the "E-11A Battlefield Airborne Communications Node", fitted out by Northrop Grumman and featuring a set of antenna fairings so much like those of the ASTOR as to make the two platforms hard to tell apart. The E-11A accepts communications from a wide variety of different radio systems and performs relay and translation among them. Four were acquired, the first being based on a Global Express 5000 and going into service from 2011, the other three being Global Express 6000 machines, with final delivery in 2013.
The Air Force is considering more orders for the platform. The crewed E-11A is complemented by the "EQ-4B BACN" version of the Northrop Grumman Global Hawk drone, with the E-11A performing fast-reaction missions while the EQ-4B takes on endurance missions.
Raytheon is still promoting special mission variants of the Global Express similar to ASTOR, likely with alternate payload configurations -- for example, a maritime patrol platform with AN/APY-10 sea-search radar instead of the ASARS-2. An electro-optic day-night imager would be another useful option. Existing communications, datalink, and self-defense kit could be leveraged into such new variants. So far, nobody has bought them, or at least hasn't admitted to have bought them; aircraft spotters have noted a number of Bombardier airframes, not known to be associated with BACN or other overt military programs, sporting radomes and other unusual kit.
In early 2015, Israel Aircraft Industries (IAI) announced a maritime patrol aircraft, the "ELI-3360", based on the Global Express 5000. It was fitted with the IAI ELM-2022 maritime search radar; an electro-optical/infrared sensor turret; an ELL-8385 electronic support measures/electronic intelligence suite; electronic warfare and self-protection equipment; plus a comprehensive communications fit comprising radios, datalinks and satellite communications. An integrated multi-mission command and control suite included operator workstations and a weapon and stores management system for under-wing stores, including homing torpedoes and anti-ship missiles.BACK_TO_TOP
* I had wanted to write up materials on business jets for a long time but never got around to it. Once I started hanging around airports and taking pictures of airliners, I ended up with some nice images of Regional Jets and the like that I really had to make use of. I also had to write this up because I was thoroughly confused about the Challenger / Regional Jet / Global Express family, and needed to sort them out.
* As concerns copyrights and permissions for this document, all illustrations and images credited to me are public domain. I reserve all rights to my writings. However, if anyone does want to make use of my writings, just contact me, and we can chat about it. I'm lenient in giving permissions, usually on the basis of being properly credited.
* Sources include:
I obtained much of the stats off the Bombardier website. I originally picked up some materials off the general Web, thinking I could render them down to speed up writing this document, but the result was the opposite. The web materials were littered with careless errors and I had to go over the stats carefully to weed them out.
* Revision history:
v1.0.0 / 01 jun 06 v1.0.1 / 01 mar 08 / Review & polish. v1.0.2 / 01 feb 10 / Review & polish. v1.1.0 / 01 jan 12 / Added section on ASTOR & military variants. v1.2.0 / 01 dec 13 / BACN comments, CS100 first flight, Boeing MSA. v1.3.0 / 01 nov 15 / Challenger 350 & 650, GX 7000 & 8000, ELI-3360, etc.BACK_TO_TOP